Hi,
It may be nothing to do with gsm nor pstn. You've got trouble alright, but are you sure that the rest of the system is not giving you the problem rather than the last link in the chain ? I've assumed the same thing many times and it's turned out to be a fault from a different part of the installation entirely. It might be worth checking the obvious, covers replaced correctly, no blown fuses, and so on before concluding that the problem is definitely to do with gsm.
Take Care.