Call Forwarding in SIP

This article arises from a problem we face today as how does SIP stack take care of Call Forwarding.

Suppose A is calling B and B wants to enable Call Forwarding to Caller C.

Our main issue is: Whether SIP stack at A would come to know that the call has been forwarded from B to C or would it be transparent for stack running on A.

Some basics:

Call Forwarding lets you forward your incoming wireless calls to another phone number. Airtime charges, plus any applicable long distance and/or roaming charges, are incurred for calls forwarded via the wireless network.

With Call Forwarding activated:

• You can continue to make outgoing calls from your telephone. Incoming calls will be forwarded.
• When the telephone number to which your calls are being forwarded is busy, callers to your number will receive a busy signal.
• To change the number to which your calls are forwarded, deactivate the feature. Then, follow the activation instructions to enter the new number.

Our problem got resolved from the tutorial on http://www.tech-invite.com/Ti-sip-service-7.html
It clearly indicates that

1. A signaling message would be sent to client, i.e. 181, Call is being forwarded.
   
2. No response message is sent from the client side to the proxy for this 181 message.
3. Rest of the signaling messages received by the client are same, i.e. 180 Ringing, 200 Okay.
   

Hence, Call forwarding at the destination does not really matter to the originating client from protocol signaling aspects.