No IVR while dial number is offline

Hi,
I’m running Asterisk 11.2.1 with a2billing. Everything is fine and working, but while I’m dialing to any number which is offline its replying nothing and waiting as

for while and terminate call with

Got SIP response 480 "Temporarily Unavailable" back from 96.xxx.1xx.138:5060 -- SIP/bd-trunk-00000028 is circuit-busy

Does anybody have such experience ? Any solution for this ??

Thanks.

That is a normal behavior, if you want to playback custom messages you need to use the HANGUPCAUSE variable and play the message according to that value.