[Asterisk-Users] Changing caller id on transfer

[Cosmin Prund]

As usual, this is most likely a easy question, but here it goes any way:

How can I change the caller id on a transferred call so the called party
knows the call has been transferred from a colleague and it's not coming
directly from our outside lines?

The story goes like this:
1) Client calls. All phones ring.
2) Someone picks up the phone.
3) The phone gets transferred to someone.
4) The person that gets the transferred call sees the original caller id and
doesn't know the call has been transferred. I'd like the person that gets
the transfer to see the caller id with a digit prefix. Ex: Original
caller-id: 0269123456; Caller id if the call has been transferred:
1*0269123456

I know I can use SetCallerId(1*${CALLERIDNUM}) but how do I know I'm doing a
transfer and not calling someone?