I currently utilize Next Alarm (years now) with a monitored VOIP line and it fails over to a cellular line with a combo cellular modem here.
The cellular modem is a back up Internet and cellular line connection. The main house VOIP line failover to the cellular modem if primary ISP goes down.
I see people do that, but I myself wonder the logic of it.
So you are using two methods of transport, IP and cellular. Since cellular is very reliable, and IP is less so, why don't you use cellular FIRST and then IP as a backup? During a fire, lets say, there are many reasons IP could go down. Lets say it does. Now your system has to "try" IP until it determines its down, then switch over to cellular. I'm not sure how fast this happens, but its got to take some time, and this is time when fire trucks could be on the way.
Most systems only test once a week, so there is not a whole lot of cellular calls actually being made, and you need to have the cellular line in place all the time, and that is the cost.
So what is the advantage of IP then only switching to cellular when IP is down? Why don't you use cellular first, then only use IP if cellular is down, and since cellular is almost NEVER down, that won't happen often. Going the other way is marginally cheaper for your monitoring company, but not an advantage for you.
I only use cellular. Like I say, I monitor it constantly and have only seen an outage once in several years in the middle of the night, for less than an hour. Chances are, if cellular is down, there is a hurricane or wildfire, and police wouldn't be able to get to your home at that time anyway.