xerox workcentre 7855 will not receive fax from Specific number

We have a xerox workcentre 7855 at one of our offices. It sends and recieves faxes with no issue except for one specific number. It is one of the county offices and we can send a fax to them with no issue, issue is with receiving fax from this number. One of the agents at this office states that they can hear the fax coming in but then it appears the fax transmission just stops and nothing is received. The county office get a confirmation page when they send to us even though the fax is not received. Called the service provider who tested the line and found no issues. Any ideas? Thanks