Hypothetical question for the Toshiba experts?

A customer has a 424 B with a 4-wire EM T1 service into it, with a standard setup for DID users. They have included their fax server and fax machine on 2 separte analog station ports.

Outgoing fax works fine, no issues; however, inbound is a different story. Intermitantly the fax transmission is interrupted and only partial info is avaliabe on the fax.

If memory serves a 4-wire EM T1 is voice grade only and although a fax can work over voice-grade POTS service it was never supported over T1.

Here is the real question:

Is it possible that an incomming fax is presented to the 424, it signals the CO that it is ready to receive and the call is presented; however, during the transmission of the fax the system mis-interprits the various and specific tones received as an FSK signal for disconnect supervision and shuts the port down thinking the call had been completed. Meanwhile the CO still has the call nailed up because it has not received the disco signal from the originating end?
The caller shows the fax complete; however, the 424 had dropped the connection and only partial information was presented to the fax.

I guess an easier question would be, are fax transmissions supported over an analong station card with 4-wire EM T1 services on a 424?
Is it supported using PRI, and are PRI services supported on a 424

Thank you in advance for any assistance