Call Forwarding Busy Line lets you forward your calls to a pre-programmed number such as an answering service whenever your phone line is busy.  You can use this service with regular call forwarding, but when regular call forwarding is activated, call forwarding busy line will be deactivated.

The pre-programmed number you designate is normally pre-set and cannot be changed at will.

Some telephone companies, especially VoIP providers will allow you to modify the forwarding number through a web portal or by dialing a *xx code.