My thoughts on this are to have a battery backed GPRS capable router in a secured location. This offers better future proofing, as it's not just the alarm that needs to keep communications up if there's a broadband / phone line issue. There's CCTV, HA and more. It's more efficient to have an unlimited data SIM, rather than 3 separate SIM's to track and pay for. It's also easier to keep up with the latest tech. I'm already 5G capable - just waiting on my area being upgraded.
If your hone network has its own failover, then all you need is polled monitoring with a decent polling interval to alert you of a missing heartbeat.