Have you used the PMS Emulator app from Mitel to test at the iPocket if it is talking properly?
Typically you have 3 interfaces from a PBX to PMS, a PBX interface for check-in, check-out, moves, DND, etc., a Voicemail interface that gets check-in/check-out information for guests (it does NOT get that from the PBX interface), and a SMDR interface that interfaces to either the PMS directly if it costs calls, or to a call cost accounting unit such as a Tel Electronics Inn Form XL.