Polycom ATA Fax

Hello, We are using a Polycom ATA connected to a MFP, we can fax from the MFP to a RC end-user, as well as to a traditional fax machine. Our ATA is configured on a limited license. When faxing to this ATA from either RC end-user or a traditional fax machine the MFP never receives incoming fax…receive confirmation that fax has been sent from sending device. When direct number is dialed, we can hear the fax tone. When a POTS line is connected to MFP works fine…All thoughts welcome, Thanks in advance!!