To clarify, the GSM box is connected to the FXO port of the UC500? What type of GSM box is this by the way?
What is the call flow that fax calls will take? Are you just trying to get faxes to work from the FXO port to an FXS port that the fax machine is connected to?
If you could, attach your configurations with all the passwords removed and we can take a look at it.
Thanks,
Anthony