183 progress

180 Ringing

followed by:

183 Progress

Typically in such a case, the 183 Progress is followed by early media, either consisting of ringback tone, or an error indication, which may be a tone, or a voice announcement. These tones will be forwarded to the caller, but not interpreted by Asterisk. The call will stay in an early media state until either ISP2 sends a final status, or the caller cancels the call.