Petey,
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.
Thanks,
Justin McAteer
Out of the Blocks Consulting, LLP