I like the alarms with proximity sensor and verbal warning (ampified voice chip): "WARNING! You have activated the vehicle alarm first stage, back away from the vehicle. Do not approach any closer or touch the vehicle or the full alarm will be activated and authorities notified."
If you use a pager notification type system be sure you know the exact location you are at and are carrying... Not gonna help to call the police: " ...it's parked in a big lot next to a motel, I think it's a holiday inn, on a major street." Or to approach 3 or 4 thugs while armed with just a cell phone.