This article describes how to use Asterisk, FreePBX, or AsteriskNOW to forward calls by using the your telephone company’s *72 / *73 feature via analog FXO (POTS) channels. This functionality can be useful for forwarding calls to an after-hours answering service without having to dial out on a separate line and bridging the calls together inside of Asterisk. Bridging calls together will often result in a dB loss within the phone system and will tie up two lines per simultaneous conversation. An added advantage is that the correct caller-id will pass through to the call recipient.
Please note that due to the nature of analog channels, there is no way to account for the risk of “glare” — a situation where the phone system attempts to dial at the same moment as an inbound call occurs, so be sure to test to make absolutely sure that the phones are actually forwarded or unforwarded each time you use this methodology.