Intermittent issue with double digits in extensions

We have an AsteriskNOW system that uses a Digium DAHDI board connected to the outside world. Our system has a small number of users (about 25) with a few of the extensions containing repeat digits (i.e. x3366). Periodically we have a caller from the outside who is unable to successfully dial those extensions once the system answers and they are in the IVR. Investigation seems to indicate that the system is not interpreting the double digits correctly. Anecdotally it also seems that this is generally associated with calls originating from certain phone services (i.e. AT&T cell phones, for example). I’m not a full time system administrator or phone administrator, just an engineer at a small business who has had a 99% positive experience with the Asterisk/FreePBX product providing phones for our employees. Unfortunately that makes debugging these 1% issues problematic for me since I really don’t know where to start other than Google or these forums. I’m hoping someone can point me to the correct configuration to update which will allow these callers to have success in reaching the desired party.