I recently had troubles with modems and faxes on SCCP-controlled VG224s and VG310s. In the VG config under the "voice service voip" section I had to add modem passthrough nse codec g711ulaw. Once I added that, I had no further issues with modems, postal machines, etc. Also, in the same section, I added "fax protocol pass-through g711ulaw" and had no further issues with faxes.
I hope that helps.
Bill