I would look for a different provider and if thats not an option, I’d host what you can so you have data on where things are failing. Now a days, I’m finding there are too many points of failure in services which means even more passing of the buck, as they say.
I’ve always understood (for the UK dont know what its like outside the UK), text messages have to be sent within 24hrs, so with that in mind I wouldnt rely on txt messages if they are time critical. What I would do is use something like Freeswitch or Asterisk to ring the number and play a prerecorded message as you will know instantly if someone has got the message. Txt is unreliable imo because of the 24hr delivery window over the network.
Now if you need to do text to speech, Freeswitch has a good engine where you can write a report, convert it to mp3 and play it to a number that answers. TTS - FreeSWITCH - Confluence
You need to look at ATA’s gateways Gateways - FreeSWITCH - Confluence these are the devices that port the call from the computer program to the phone line, like a fancy modem, but make sure you lock this all down as this is an attack vector. Personally, I’d have a dedicated machine running Freeswitch based on past experience, to increase the isolation from the rest of the network, controlled by a firewall between the rest of your network and the freeswitch server. You can run freeswitch on a cheap raspberry pi and you can get rack mounts for multiple raspberrypi’s Raspberry Pi Rack Mount: 5 Best Racks for Pi Clusters | SHB (smarthomebeginner.com) to maintain that physical isolation from hackers which cloud servers cant always give you especially when thinking about Hertzbleed Attack and a bit of remote memory corruption like this Notes on OpenSSL remote memory corruption – Guido Vranken
and if you want to automatically handle the call at the other end, you can sent up freeswitch to handle the other end, record the call to mp3 and then speech to text it. But if you are going this route, you might just as well use DTMF tones to pass data!
Interactive Voice Response is very easy on freeswitch IVR - FreeSWITCH - Confluence.
The main point is text is not reliable, only voice calls, so maybe look at a monitoring system that uses the voice part of the telecom’s network.
The other thing is, dont underestimate the extremism within some parts of society to monitor everything that goes on, its got a fervour that’s on a par with religion in my experience, so hacking is to be expected from even the most unlikely, trusted parts of life.