When my phone system is broken you still get a ring on the callers line....
Your alarm calls a SMS Gateway, it may turn many turns w/o succesess & be quite pricey on billing
the provider of the SMS gateway gets no income, there are a legacy service & many networks turned there's off years ago...
Its comparable to a carrier pigeon, the day you really need it to work it gets eaten by next doors cat
PTSN is dead, all the copper in the ground is getting moved to fibre & the network is going VOIP, which mean modems in burglar alarm & faxes (remember them ?) won't work as the timing & tones is really critical to signalling
You need to signal via IP or a SIM card, better stuff uses both.