Controling parallel call

hi am new to asterisk

 i have 3  three number's(ie 123,456,789).
if 123 and 456 are talking ...

if 789 is trying to call 123 0r 456

i should get message like "user is busy"  how to do this any idea ???

The Dial application can be configured to jump to specific priorities in your dialplan when it encounters busy, unavailable, etc, extensions.

See the Dial command options at Voip-Info for more information.

The free book Asterisk, the Future of Telephony is a very good place to start.