I have been doing some research and it looks like some people have been successful in changing their ADT system to another company. Sounds like the issue is finding the installer code. Having found anything on monitoring ones own system thou.
I work for an alarm company, and I can tell you that if you want to reprogram the dialout numbers in your system you will need the installer code and an installer's manual for the system.
Without the installer code, some systems can be reset to factory defaults, but this also requires information from the manufacturer of the system. It also means that the system will have to be reprogrammed from scratch. User codes, delays on zones, any special programming features, etc. will all be lost and need to be reprogrammed.
Probably the cheapest answer would be to buy some kind of auto-dialer that you can program to your pager number and connect it to the siren output of the alarm panel. You may need a 12 volt relay between them depending on the auto-dialer.