Based on ths description, the VG224 is sounds like it is OK - the problem is with the fax traffic getting from your main gateway to the VG224, it is not an issue with traffic from the VG224 towards the main gateway.

Most common reason this happens - clock slips on the PSTN gateway digital trunks.

Assuming you have some kind of T1/E1 gateway, do a show controller and check the stats - if you see clock slips then this could be the problem.

The reason why the faxes work VG224 ---> PSTN is the traffic in this direction does not experience slips, but the traffic PSTN ---> VG224 has slips in the bit stream, which cause errors in the data, the fax protocols detect these erorrs and retransmit, but the errors keep occuring and eventually the fax machine drops the call.

Assuming you have a T1 card in slot 0/0/0 of the PSTN gateway router, you need the following commands -

network-clock-participate t1 wic 0

network-clock-select 1 t1 0/0/0

this will recover the clock signal off the PRI on controller e1 0/0/0 and use it to drive the DSP's and the traffic in the TX direction, so there will be a common clock reference system wide and the slips should be cleared.

If this does not help, then tell us more about the setup of the entire network and how it interfaces to the PSTN.