The best solution really is the TG-1 Express. It is UL Listed so Del is happy, but more importantly its ONLY cellular. In other-words, it doesn't need any logic to decide between IP or cellular. It uses NO wiring except for the short wire between the panel and unit, so the cutting/burning of a phone/IP cable is moot. Is cellular 100.00% up, no, but its pretty close. Cell towers are required to have battery backup and many have generator backup, as they are considered life safety devices. Overall, cellular only is going to be just as reliable, if not more so than cellular and IP combined, which as we have shown here is not all that reliable.
The TG-1 Express that Alarm Relay uses is AT&T, but if your AT&T signal is poor, I believe they make a TG-1 which is CDMA for Verizon, but I'm not sure Alarm Relay offers that model, but they probably can get it.
The TG-1 maintains a constant data connection between TG-1 unit and Telguard, and if that connection is broken, the TG-1 triggers a relay which can be monitored by the panel. In the several months that I have owned it, that connection has not been dropped even once. The TG-1 does not simply make a cellular call when your alarm goes off, its in contact with the monitoring station continuously.
The only negative of all this is cost, since someone has to pay for the data between the TG-1 and Telguard, which is greater than a modem that only connects on occasion, but you get what you pay for.