Is the alarm system there or just the wiring? Ours has the wiring only and we've chosen not to get the alarm. We interviewed three or four companies when we first moved in and they claimed they would all have to put their stuff in. After talking to a few pushy salespeople and being shocked by the prices, we opted to delay it. Brand new house then -- 14 years ago.
Check with your local firehouse. Ours do it for much cheaper
I use a company called myvirtualmonitoring.com. You do not need a phone line. Also, you can choose to self-monitor meaning it will notify a cell phone via text or whatever you like. You also have the option to have it monitored by a Central Monitoring Station which is UL certified which is important if you want to save on your homeowners insurance. I used to be with one of the National companies paying around $35/month now I pay just under $15 for the same service with the expection that I maintain my system now but their tech support has been great about helping me via phone or email.
DH and I choose not to have our alarm monitored. Ours still sounds the alarm if a door or window is opened once we arm the system, it just won't "call" anyone. If the alarm goes off at 2 AM I'll just call 911 myself. If it goes off when we aren't home, we'll just have to hope the high pitched VERY loud "beeeeeeeeeeeeeeep" scares the person off. If not and they steal something...well that's what home owners insurance is for!