Make sure to let them know ahead of time you don't have a phone line, that your "phone" service is through your cable just so it's not an issue at time of installation. Also get as many window stickers and yard signs as you can, sometimes the installers forget to leave any. It's funny how everyone wants signs/stickers, even if they don't have an alarm. I work for a division of ADT and therefore have to drive an ADT van. I can't count how many people have made up stories trying to get signs from me, they've even stopped at my house and knocked on my door trying to get some! They are ridiculous sometimes. Fortunately/unfortunately, the division I work for has nothing to do with burglar alarm systems so I don't have access to that stuff. But I do have a big giant sign on 4 wheels sitting in my driveway!

As a side note, if you do get it professionally installed and monitored, run it by your home insurance company, you can possibly get a discount on your rate.