diff --git a/packages/voice/__tests__/__dataSets__/update.ts b/packages/voice/__tests__/__dataSets__/update.ts
index b6428e32..ac475ddf 100644
--- a/packages/voice/__tests__/__dataSets__/update.ts
+++ b/packages/voice/__tests__/__dataSets__/update.ts
@@ -184,6 +184,108 @@ export default [
     error: false,
     expected: undefined,
   },
+  {
+    label: 'transfer call with NCCO with userToUser',
+    requests: [
+      [
+        `/v1/calls/${callPhone.uuid}`,
+        'PUT',
+        {
+          action: 'transfer',
+          destination: {
+            type: 'ncco',
+            ncco: [
+             {
+               action: NCCOActions.CONNECT,
+               endpoint: [
+                 {
+                   type: 'sip',
+                   uri: 'xxx',
+                   standardHeaders: {
+                      'User-to-User': 'yyy'
+                    }
+                  },
+                ]
+              },
+            ],
+          },
+        },
+      ],
+    ],
+    responses: [[204]],
+    clientMethod: 'transferCallWithNCCO',
+    parameters: [
+      callPhone.uuid,
+      [
+        {
+          action: NCCOActions.CONNECT,
+          endpoint: [
+            {
+              type: 'sip',
+              uri: 'xxx',
+              standardHeaders: {
+                'userToUser': 'yyy'
+              }
+            },
+          ],
+        },
+      ],
+    ],
+    generator: false,
+    error: false,
+    expected: undefined,
+  },
+  {
+    label: 'transfer call with NCCO with User-To-User',
+    requests: [
+      [
+        `/v1/calls/${callPhone.uuid}`,
+        'PUT',
+        {
+          action: 'transfer',
+          destination: {
+            type: 'ncco',
+            ncco: [
+             {
+               action: NCCOActions.CONNECT,
+               endpoint: [
+                 {
+                   type: 'sip',
+                   uri: 'xxx',
+                   standardHeaders: {
+                      'User-to-User': 'yyy'
+                    }
+                  },
+                ]
+              },
+            ],
+          },
+        },
+      ],
+    ],
+    responses: [[204]],
+    clientMethod: 'transferCallWithNCCO',
+    parameters: [
+      callPhone.uuid,
+      [
+        {
+          action: NCCOActions.CONNECT,
+          endpoint: [
+            {
+              type: 'sip',
+              uri: 'xxx',
+              standardHeaders: {
+                'User-to-User': 'yyy'
+              }
+            },
+          ],
+        },
+      ],
+    ],
+    generator: false,
+    error: false,
+    expected: undefined,
+  },
   {
     label: 'transfer call with NCCO',
     requests: [
diff --git a/packages/voice/lib/types/Endpoint/SIPEndpoint.ts b/packages/voice/lib/types/Endpoint/SIPEndpoint.ts
index 1bc11069..fc8e673d 100644
--- a/packages/voice/lib/types/Endpoint/SIPEndpoint.ts
+++ b/packages/voice/lib/types/Endpoint/SIPEndpoint.ts
@@ -28,7 +28,12 @@ export type SIPEndpoint = {
      *
      * @link https://tools.ietf.org/html/rfc7433
      */
-    userToUser: string;
+    userToUser?: string;
+
+    /**
+     * Allows using the pure header that is expected in the API call.
+     */
+    'User-to-User'?: string;
   }
 };
 
diff --git a/packages/voice/lib/voice.ts b/packages/voice/lib/voice.ts
index 63b8e514..0a7bf809 100644
--- a/packages/voice/lib/voice.ts
+++ b/packages/voice/lib/voice.ts
@@ -12,9 +12,11 @@ import {
   TalkAction,
   OutboundCall,
   CallWithNCCO,
+  SIPEndpoint,
 } from './types';
 
 import { ResponseTypes } from '@vonage/vetch';
+import { NCCOActions } from './enums';
 
 const apiCallsToCalls = (call: CallDetailResponse): CallDetail => {
   delete call._links;
@@ -30,6 +32,34 @@ const apiCallsToCalls = (call: CallDetailResponse): CallDetail => {
   } as CallDetail;
 };
 
+const NCCOToApiCalls = (ncco: Action[]): Array<Action> => ncco.map((action) => {
+  switch (action.action) {
+  case NCCOActions.CONNECT:
+    return {
+        ...action,
+        endpoint: action.endpoint.map((endpoint) => {
+          switch (endpoint.type) {
+          case 'sip':
+            return {
+              type: 'sip',
+              uri: endpoint.uri,
+              headers: endpoint.headers,
+              standardHeaders: {
+                'User-to-User': Object.hasOwn(endpoint.standardHeaders || {} ,'User-to-User')
+                  ? {...endpoint.standardHeaders}['User-to-User']
+                  : endpoint.standardHeaders?.userToUser,
+              }
+            } as SIPEndpoint;
+          default:
+            return endpoint;
+          }
+        })
+      };
+  default:
+    return action;
+  }
+});
+
 type NCCODestination = {
   type: 'ncco';
   url?: Array<string>;
@@ -443,10 +473,14 @@ export class Voice extends Client {
    * ```
    */
   async transferCallWithNCCO(uuid: string, ncco: Action[]): Promise<void> {
-    return this.callAction(uuid, 'transfer', {
-      type: 'ncco',
-      ncco: ncco,
-    });
+    return this.callAction(
+      uuid,
+      'transfer',
+      {
+        type: 'ncco',
+        ncco: NCCOToApiCalls(ncco),
+      },
+    );
   }
 
   /**