Ok phone line, is internet to me since it is either VOIP or, when it isn't, it would be ADSL which goes through the phone line. These days they are one and the same. The fallback to cellular data can be done through your router. So you can get notifications and SMS from the vera even if my internet/phone line is disrupted. You can also get notification from the router when my ISP has an interruption.
The question being why pay a monitoring company to call you or whoever you want when you can have your home call you directly?
The verification that it isn't a false alarm is where it fails. It typically takes too long. And when they would call me, how did I know whether it was a false alarm? Unless I am home, which then means the monitoring is not needed, I have to check my system to see which sensors were triggered and check my cameras. Sometimes I would have to call neighbors to go take a look before confirming. I can then decide whether or not to call law enforcement myself, why would I need to ask the monitoring company to make the call for me? The notifications can go to multiple people just the same. My opinion the alarm monitoring is that in best cases, it is equivalent to a self monitoring with fallback and most of the time an expensive annoyance. It may buy you some insurance discount however.
The OP is asking how to setup a fallback for notifications in case of internet failure. My answer is SMS using a router with a cellular fallback. One can find cheap ($20) cellular modems with prepaid cellular SIM. The dual WAN routers vary a lot in price. I think I got a cradlepoint for ~$80.