Being IAX specific suggests an IAX problem. You need to provide verbose console log traces, initially, and possibly detailed IAX and DAHDI debug logging, later.
Answer and disconnect supervision applies to everything. Analogue channels often lack it. Common channel signalling is pretty much certain to have it. However, PRI, on its own, implies channel associated signalling, which may well not have it.http://www.cisco.com/en/US/tech/tk652/t ... 2560.shtml
contains some information on T1 robbed bit signalling options, and specifically says supervision may not be available.