I see people referring to jam proof wireless alarms and I don't believe there can be any such thing. You get a transmitter powerful enough, if it is wireless, it can be jammed.
Systems can utilize a sensor heartbeat that makes the sensors check in with the control panel periodically, and if they aren't heard from then panel can trigger and report a trouble, or even be programmed to report it as a zone violation, but things can still be jammed.
Encrypting the signal is helpful in that someone can't create a signal that emulates a given sensor and makes it look like the zone hasn't been violated when it has, but at the end of the day, if it is wireless it is susceptible to tampering.
I prefer hard wired alarm systems and only utilize wireless sensors when absolutely necessary.
As for monitoring, I have had good luck with Alarmrelay. I have used them for years on 2 properties with my self installed DSC systems. http://www.alarmrelay.com/