Call Forwarding and Call Bridging

In my laymen’s view of phone sysetms there are two different ways to connect people together in a call:

Call Bridging: User A dials PBX, PBX dials User B, PBX connects A and B together. This is a standard conference call. The key thing here is that 2 lines of the PBX are used while the call is taking place.

Call Forwarding: User A dials User B, but number is re-routed to some number other than the one listed for User B. This is call forwarding as provided by the phone company.

My question is, can Asterisk provide TRUE call forwarding. That is after making the connection, the Asterisk system no longer has lines tied up in the conversation. It would seem to me this is possible, as service like 411 do this (or maybe they don’t, maybe they just bridge you together). But most information on call forwarding deals with extensions within the office, and forwards to outside numbers consume line resources or so it seems.

Is there Signalling messages that can be sent to the CO to link the lines together? Or is this something only the phone company can do?

Thanks for any information

Being that call goes through asterisk you will need to use two channels. The phone company does it well they just never told you :wink:


If you are referring to transferring a call from one external line to another, there are a few ways to do it, depending on what type of external connections you have.

If you have analog lines connected to your Asterisk server, you can implement this with a ‘Flash Hook Transfer.’ This is analogous to how you would use three-way calling on an analog line. In fact, you will need to have three-way calling enabled for this to work.

If you have a ISDN-PRI (digital) trunk, you may be able to use a feature called “two B-channel transfer” to implement what you would like.

If you have SIP trunks, this would be implemented using a RE-INVITE.

Justin McAteer
Out of the Blocks Consulting, LLP