The number you dialed is currently unavailable

we are using asterisk 1.2.29 bristuffed 0.3.0-PRE-1y-s with freepbx for a handfull of our customers. Almost evereything is fine.
But we would like to have more specific result codes / messages for calls where we usualy get "everyone is busy/congested … please try…"
e.g. we would like to have a message like “the number you have dialed is currently unavailable …” which we get, when we call this number with a mobile phone but which we do not get when we call from our asterisk / freepbx.

Any help will be appreciated.

Kind Regards,