https://wiki.asterisk.org/wiki/display/AST/Feature+Code+Call+Transfers

In Asterisk, a user can transfer a phone call. Transferring a call involves redirecting an existing call to a new location or user. This can be done using two different transfer methods, a blind transfer or an attended transfer.

The first transfer method is the blind transfer. Using this method you can dial a special number to initiate the transfer, then dial the extension to transfer the call, and hangup. In the blind transfer there is no verification that the phone call was answered by the user at the destination. This is why it is called a blind transfer.

The second transfer method is the attended transfer. With this type of transfer you can dial a special number to initiate the transfer, then dial the extension to transfer the call. At this point the destination extension will be called. This gives you the ability to talk with the destination user and tell them who may be on the line being transferred. Now when you hang up, the transfer will complete automatically.