fix #1444: problem of client registration at Yeastar SIP server (#1445)

Author: mjalil

examples/Softphone/Softphone.xaml.cs

@@ -132,7 +132,8 @@ private async Task Initialize()
                 m_sipUsername,
                 m_sipPassword,
                 m_sipServer,
-                REGISTRATION_EXPIRY);
+                REGISTRATION_EXPIRY,
+                sendUsernameInContactHeader: true);
 
             _sipRegistrationClient.Start();
         }