Allow customisation of the call descriptor when transferring calls (#1161)

Author: henlil

src/app/SIPUserAgents/SIPUserAgent.cs

@@ -1,4 +1,4 @@
-//-----------------------------------------------------------------------------
+//-----------------------------------------------------------------------------
 // Filename: SIPUserAgent.cs
 //
 // Description: A "full" SIP user agent that encompasses both client and server 
@@ -325,6 +325,15 @@ public SIPURI ContactURI
         /// </remarks>
         public event Func<SIPUserField, string, bool> OnTransferRequested;
 
+        /// <summary>
+        /// Allows the call descriptor for the new call to be customised before a transfer.
+        /// </summary>
+        /// <remarks>
+        /// SIPCallDescriptor: The call descriptor to be customised
+        /// SIPRequest: The refer request that initialised the transfer
+        /// </remarks>
+        public event Action<SIPCallDescriptor, SIPRequest> OnTransferCallDescriptorCreated;
+
         /// <summary>
         /// Fires when the call placed as a result of a transfer request is successfully answered.
         /// The SIPUserField contains the destination that was called for the transfer.
@@ -1218,6 +1227,8 @@ private void ProcessTransferRequest(SIPRequest referRequest)
                                null,
                                null);
 
+                            OnTransferCallDescriptorCreated?.Invoke(callDescriptor, referRequest);
+
                             var transferResult = await Call(callDescriptor, MediaSession).ConfigureAwait(false);
 
                             logger.LogDebug($"Result of calling transfer destination {transferResult}.");