I have a problem with fax in one of my site. I'm using MGCP for fax protocol. The faxes can send and receive faxes using extension. They also can send faxes through PSTN, but they cannot receive faxes from PSTN. This problem happens in only certain faxes (in this case Xerox Document Centre). I've tried to change the fax settings one by one, but it didn't give any positive results. Why this can happen ? Are there any certain types of fax that are not supported by Cisco ?