Planning confusion? Vista 20p and communication

I have once installed a Vista 20p and RF keypad, that was a few years ago. I would like to stick with the Vista 20P as I somewhat familiar with it. What has me confused is communicating. I do not have a landline phone and would like to use cell phone technology (GSM)? This is not for calling my cell phone, but for a monitoring service. 1. What should I buy or what would be best, to be able to communicate without a landline? 2. If I buy it will I be locked in to some kind of Honeywell monitoring? My last system was unmonitored and I keep reading but I am in a fog on this. JIm