Sounds like the Voice Mail is working. The guests are already checked in, and the PMS has not checked them out, that's why they are working. PMS people can be awfully stubborn in their attempt to place blame on the PBX vendor, it never seems to be their (PMS) fault, but it usually is. See if they can re-set the voice mail port on their side, or move to a different port on the interface (octopus cable). Good luck.