Can I reprogram an ADT alarm system to be used “unmonitored” or to call my cell phone instead?
I moved into a new house that has the ADT system already installed, I don’t want to pay for the monitoring service but I would like to have it functional and to call my cell phone intead of the ADT Central

The call would come from the monitoring desk. ADT and other home security systems have their own reserved frequencies that you can’t tap into unless you’re some radio genius geek, but you would have figured that out already.
The previous answerer is a little off. If this is the typical ADT DACT which transmits over telephone lines, it is possible to reprogram the DACT to call any number (and sometimes numbers) you wish. Problem is to get instructions on how to do the programming. I might have some help for you if you contact me privately. I am not in that business, but know a goo bit about it.
A question would be whether ADT sold the system, or was leasing it to the previous owner.