diff --git a/client/custom_httpclient_test.go b/client/custom_httpclient_test.go
new file mode 100644
index 000000000..7e3f0f2d5
--- /dev/null
+++ b/client/custom_httpclient_test.go
@@ -0,0 +1,135 @@
+package client_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/twilio/twilio-go/client"
+)
+
+// TestCustomHTTPClientUsed tests that a custom HTTPClient is actually used for requests
+func TestCustomHTTPClientUsed(t *testing.T) {
+	// Track if custom transport was used
+	customTransportUsed := false
+
+	// Create a custom transport that tracks usage
+	customTransport := &http.Transport{
+		Proxy: func(req *http.Request) (*url.URL, error) {
+			customTransportUsed = true
+			// Return no proxy, just track that this was called
+			return nil, nil
+		},
+	}
+
+	// Create http.Client with custom transport
+	httpClient := &http.Client{
+		Transport: customTransport,
+	}
+
+	// Create mock server
+	mockServer := httptest.NewServer(http.HandlerFunc(
+		func(writer http.ResponseWriter, request *http.Request) {
+			writer.WriteHeader(200)
+			writer.Write([]byte(`{"status":"ok"}`))
+		}))
+	defer mockServer.Close()
+
+	// Create base client with custom HTTPClient
+	baseClient := &client.Client{
+		Credentials: client.NewCredentials("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"),
+		HTTPClient:  httpClient,
+	}
+	baseClient.SetAccountSid("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+
+	// Test sending a request through the client
+	resp, err := baseClient.SendRequest("GET", mockServer.URL+"/test", nil, map[string]interface{}{
+		"Content-Type": "application/x-www-form-urlencoded",
+	})
+
+	assert.NoError(t, err)
+	assert.NotNil(t, resp)
+	assert.Equal(t, 200, resp.StatusCode)
+	
+	// Verify custom transport was used
+	assert.True(t, customTransportUsed, "Custom HTTPClient transport should have been used")
+}
+
+// TestCustomHTTPClientViaRequestHandler tests that custom HTTPClient works through RequestHandler
+func TestCustomHTTPClientViaRequestHandler(t *testing.T) {
+	// Track if custom transport was used
+	customTransportUsed := false
+
+	// Create a custom transport that tracks usage
+	customTransport := &http.Transport{
+		Proxy: func(req *http.Request) (*url.URL, error) {
+			customTransportUsed = true
+			// Return no proxy, just track that this was called
+			return nil, nil
+		},
+	}
+
+	// Create http.Client with custom transport
+	httpClient := &http.Client{
+		Transport: customTransport,
+	}
+
+	// Create mock server
+	mockServer := httptest.NewServer(http.HandlerFunc(
+		func(writer http.ResponseWriter, request *http.Request) {
+			writer.WriteHeader(200)
+			writer.Write([]byte(`{"status":"ok"}`))
+		}))
+	defer mockServer.Close()
+
+	// Create base client with custom HTTPClient
+	baseClient := &client.Client{
+		Credentials: client.NewCredentials("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"),
+		HTTPClient:  httpClient,
+	}
+	baseClient.SetAccountSid("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+
+	// Create RequestHandler with the base client
+	requestHandler := client.NewRequestHandler(baseClient)
+
+	// Test sending a request through the RequestHandler
+	resp, err := requestHandler.Get(mockServer.URL+"/test", nil, map[string]interface{}{
+		"Content-Type": "application/x-www-form-urlencoded",
+	})
+
+	assert.NoError(t, err)
+	assert.NotNil(t, resp)
+	assert.Equal(t, 200, resp.StatusCode)
+	
+	// Verify custom transport was used
+	assert.True(t, customTransportUsed, "Custom HTTPClient transport should have been used through RequestHandler")
+}
+
+// TestDefaultHTTPClientCreatedWhenNil tests that a default HTTPClient is created when nil
+func TestDefaultHTTPClientCreatedWhenNil(t *testing.T) {
+	// Create mock server
+	mockServer := httptest.NewServer(http.HandlerFunc(
+		func(writer http.ResponseWriter, request *http.Request) {
+			writer.WriteHeader(200)
+			writer.Write([]byte(`{"status":"ok"}`))
+		}))
+	defer mockServer.Close()
+
+	// Create base client WITHOUT custom HTTPClient (nil)
+	baseClient := &client.Client{
+		Credentials: client.NewCredentials("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"),
+		HTTPClient:  nil, // Explicitly set to nil
+	}
+	baseClient.SetAccountSid("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+
+	// Test sending a request - should create default HTTP client
+	resp, err := baseClient.SendRequest("GET", mockServer.URL+"/test", nil, map[string]interface{}{
+		"Content-Type": "application/x-www-form-urlencoded",
+	})
+
+	assert.NoError(t, err)
+	assert.NotNil(t, resp)
+	assert.Equal(t, 200, resp.StatusCode)
+}
diff --git a/twilio_custom_httpclient_integration_test.go b/twilio_custom_httpclient_integration_test.go
new file mode 100644
index 000000000..af9725aea
--- /dev/null
+++ b/twilio_custom_httpclient_integration_test.go
@@ -0,0 +1,127 @@
+package twilio
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/twilio/twilio-go/client"
+)
+
+// TestRestClientWithCustomHTTPClient tests that custom HTTPClient works through full RestClient flow
+func TestRestClientWithCustomHTTPClient(t *testing.T) {
+	// Track if custom transport was used
+	customTransportUsed := false
+
+	// Create a custom transport that tracks usage
+	customTransport := &http.Transport{
+		Proxy: func(req *http.Request) (*url.URL, error) {
+			customTransportUsed = true
+			// Return no proxy, just track that this was called
+			return nil, nil
+		},
+	}
+
+	// Create http.Client with custom transport
+	httpClient := &http.Client{
+		Transport: customTransport,
+	}
+
+	// Create mock Twilio API server
+	mockServer := httptest.NewServer(http.HandlerFunc(
+		func(writer http.ResponseWriter, request *http.Request) {
+			// Verify Authorization header is present (basic auth)
+			authHeader := request.Header.Get("Authorization")
+			assert.NotEmpty(t, authHeader, "Authorization header should be present")
+
+			writer.WriteHeader(200)
+			writer.Write([]byte(`{"account_sid":"ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx","balance":"100.00","currency":"USD"}`))
+		}))
+	defer mockServer.Close()
+
+	// Create Twilio base client with custom HTTPClient (following documentation pattern)
+	baseClient := &client.Client{
+		Credentials: client.NewCredentials("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"),
+		HTTPClient:  httpClient,
+	}
+	baseClient.SetAccountSid("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+
+	// Create Twilio RestClient with custom client
+	twilioClient := NewRestClientWithParams(ClientParams{
+		Client: baseClient,
+	})
+
+	// Verify the custom client was passed through
+	assert.NotNil(t, twilioClient.RequestHandler)
+	assert.NotNil(t, twilioClient.RequestHandler.Client)
+	assert.Equal(t, baseClient, twilioClient.RequestHandler.Client)
+
+	// Verify HTTPClient is accessible through the chain
+	clientImpl, ok := twilioClient.RequestHandler.Client.(*client.Client)
+	assert.True(t, ok, "Client should be of type *client.Client")
+	assert.Equal(t, httpClient, clientImpl.HTTPClient, "Custom HTTPClient should be preserved")
+
+	// Make a request through the client directly to verify custom HTTPClient is used
+	resp, err := baseClient.SendRequest("GET", mockServer.URL+"/Balance.json", nil, map[string]interface{}{
+		"Content-Type": "application/x-www-form-urlencoded",
+	})
+
+	assert.NoError(t, err)
+	assert.NotNil(t, resp)
+	assert.Equal(t, 200, resp.StatusCode)
+
+	// Verify custom transport was used
+	assert.True(t, customTransportUsed, "Custom HTTPClient transport should have been used for API requests")
+}
+
+// TestRestClientWithCustomHTTPClientAndTimeout tests that SetTimeout works with custom HTTPClient
+func TestRestClientWithCustomHTTPClientAndTimeout(t *testing.T) {
+	// Create a custom http.Client
+	customHTTPClient := &http.Client{
+		Transport: http.DefaultTransport,
+	}
+
+	// Create Twilio base client with custom HTTPClient
+	baseClient := &client.Client{
+		Credentials: client.NewCredentials("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"),
+		HTTPClient:  customHTTPClient,
+	}
+	baseClient.SetAccountSid("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+
+	// Create Twilio RestClient
+	twilioClient := NewRestClientWithParams(ClientParams{
+		Client: baseClient,
+	})
+
+	// Set timeout via RestClient
+	twilioClient.SetTimeout(30 * time.Second)
+
+	// Verify the custom HTTPClient is still the same instance (not replaced)
+	clientImpl, ok := twilioClient.RequestHandler.Client.(*client.Client)
+	assert.True(t, ok)
+	assert.Equal(t, customHTTPClient, clientImpl.HTTPClient, "SetTimeout should not replace custom HTTPClient")
+	assert.Equal(t, 30*time.Second, clientImpl.HTTPClient.Timeout, "Timeout should be updated on custom HTTPClient")
+}
+
+// TestRestClientWithoutCustomClient tests default behavior when no custom client provided
+func TestRestClientWithoutCustomClient(t *testing.T) {
+	// Create RestClient without custom client (should create default)
+	twilioClient := NewRestClientWithParams(ClientParams{
+		Username: "ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
+		Password: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
+	})
+
+	// Verify a client was created
+	assert.NotNil(t, twilioClient.RequestHandler)
+	assert.NotNil(t, twilioClient.RequestHandler.Client)
+
+	// Verify it's a default client
+	clientImpl, ok := twilioClient.RequestHandler.Client.(*client.Client)
+	assert.True(t, ok)
+
+	// HTTPClient will be nil until first request (lazy initialization via defaultHTTPClient())
+	assert.Nil(t, clientImpl.HTTPClient, "HTTPClient should be nil before first use")
+}
diff --git a/twilio_proxy_test.go b/twilio_proxy_test.go
new file mode 100644
index 000000000..4d234a62a
--- /dev/null
+++ b/twilio_proxy_test.go
@@ -0,0 +1,135 @@
+package twilio
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/twilio/twilio-go/client"
+)
+
+// TestCustomHTTPClientWithProxyURL tests the exact scenario from the issue:
+// Using http.ProxyURL with a custom HTTPClient
+func TestCustomHTTPClientWithProxyURL(t *testing.T) {
+	// Create a mock proxy server that tracks if it received requests
+	mockProxyServer := httptest.NewServer(http.HandlerFunc(
+		func(writer http.ResponseWriter, request *http.Request) {
+			// A real proxy would forward the request, but we just track it
+			writer.WriteHeader(200)
+			writer.Write([]byte(`{"status":"proxied"}`))
+		}))
+	defer mockProxyServer.Close()
+
+	// Parse proxy URL
+	proxyURL, err := url.Parse(mockProxyServer.URL)
+	assert.NoError(t, err)
+
+	// Create http.Client with proxy (EXACT pattern from issue description)
+	httpClient := &http.Client{
+		Transport: &http.Transport{
+			Proxy: http.ProxyURL(proxyURL),
+		},
+	}
+
+	// Create Twilio client with custom HTTPClient (EXACT pattern from issue description)
+	accountSid := "ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+	authToken := "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+
+	baseClient := &client.Client{
+		Credentials: client.NewCredentials(accountSid, authToken),
+		HTTPClient:  httpClient,
+	}
+	baseClient.SetAccountSid(accountSid)
+
+	twilioClient := NewRestClientWithParams(ClientParams{
+		Client: baseClient,
+	})
+
+	// Verify client setup
+	assert.NotNil(t, twilioClient)
+	assert.NotNil(t, twilioClient.RequestHandler)
+	assert.NotNil(t, twilioClient.RequestHandler.Client)
+
+	// Create a mock Twilio API server (simulating api.twilio.com)
+	mockTwilioServer := httptest.NewServer(http.HandlerFunc(
+		func(writer http.ResponseWriter, request *http.Request) {
+			// This represents the actual Twilio API
+			writer.WriteHeader(200)
+			writer.Write([]byte(`{"account_sid":"ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx","balance":"100.00","currency":"USD"}`))
+		}))
+	defer mockTwilioServer.Close()
+
+	// Make a request - should go through proxy
+	// Note: In a real scenario with a real proxy, this would work
+	// For testing, we verify the HTTPClient with Proxy is set up correctly
+	resp, err := baseClient.SendRequest("GET", mockTwilioServer.URL+"/Balance.json", nil, map[string]interface{}{
+		"Content-Type": "application/x-www-form-urlencoded",
+	})
+
+	// The request should succeed (our mock server responds directly)
+	// In a real proxy setup, the proxy would forward to the real server
+	assert.NoError(t, err)
+	assert.NotNil(t, resp)
+
+	// The key verification: the Transport with Proxy is configured
+	clientImpl, ok := twilioClient.RequestHandler.Client.(*client.Client)
+	assert.True(t, ok)
+	assert.NotNil(t, clientImpl.HTTPClient)
+	assert.NotNil(t, clientImpl.HTTPClient.Transport)
+
+	transportImpl, ok := clientImpl.HTTPClient.Transport.(*http.Transport)
+	assert.True(t, ok, "Transport should be *http.Transport")
+	assert.NotNil(t, transportImpl.Proxy, "Proxy function should be set")
+
+	// Verify proxy function returns the correct proxy URL
+	proxyURLFromTransport, err := transportImpl.Proxy(&http.Request{URL: &url.URL{Scheme: "https", Host: "api.twilio.com"}})
+	assert.NoError(t, err)
+	assert.Equal(t, proxyURL.String(), proxyURLFromTransport.String(), "Proxy URL should match")
+}
+
+// TestCustomHTTPClientProxyFunctionCalled tests that the proxy function is actually invoked
+func TestCustomHTTPClientProxyFunctionCalled(t *testing.T) {
+	proxyFunctionCalled := false
+	callCount := 0
+
+	// Create custom transport with tracking proxy function
+	customTransport := &http.Transport{
+		Proxy: func(req *http.Request) (*url.URL, error) {
+			proxyFunctionCalled = true
+			callCount++
+			// Don't actually use a proxy, return nil
+			return nil, nil
+		},
+	}
+
+	httpClient := &http.Client{
+		Transport: customTransport,
+	}
+
+	// Create Twilio client
+	baseClient := &client.Client{
+		Credentials: client.NewCredentials("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"),
+		HTTPClient:  httpClient,
+	}
+	baseClient.SetAccountSid("ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+
+	// Create mock server
+	mockServer := httptest.NewServer(http.HandlerFunc(
+		func(writer http.ResponseWriter, request *http.Request) {
+			writer.WriteHeader(200)
+			writer.Write([]byte(`{}`))
+		}))
+	defer mockServer.Close()
+
+	// Make request
+	resp, err := baseClient.SendRequest("GET", mockServer.URL, nil, map[string]interface{}{
+		"Content-Type": "application/x-www-form-urlencoded",
+	})
+
+	assert.NoError(t, err)
+	assert.NotNil(t, resp)
+	assert.True(t, proxyFunctionCalled, "Proxy function should have been called")
+	assert.Greater(t, callCount, 0, "Proxy function should have been called at least once")
+}