> I should be considering bearing in mind that it would be expensive to re-write the software to work with another vendor
Well, maybe next time you get somebody that implements a standard.
With that kind of behavior (not letting you speak to anybody, the blocking is understandable), it's clear you shouldn't keep their services. So, you have now an opportunity to do it right, and make the next move cheaper.
SMPP is a standard for sending and receiving text messages. The standard body is dissolved and you have to grab the documents from random places, but that doesn't make it less of a standard. There's also a SOAP standard for SMS that GSMA put out, but it's SOAP bleh. I think there's supposed to be a newer GSMA standard that's not SOAP garbage, but I retired.
There isn't really a standard for the interactive voice processing, SIP is a standard, but it's not quite simple to use it compared to Twilio or similar APIs.
However, most of the providers in this space have fairly simple APIs, so it's like 30 minutes of work to do the integration, maybe a little more if you're also receiving SMS or if URLs are particularly hard to use in your language of choice; if you're adding yet another SMPP vendor, that's faster, if you're adding yet another GSMA SOAP vendor, it's probably longer because you'll have to figure out why your XML doesn't work even though it should. Plus whatever it takes to get the account setup. Plus however long to build a way to choose from multiple providers (this part may be a lot of work!), and however long you want to run with limited traffic to see if the new provider does better/worse/same as the old provider.
VoIP is entirely based on SIP, everywhere. Those providers just add a proprietary API over it. But if the use case is just sending texts, that one may evade the standard (I don't remember it entirely, and it certainly evolved since last I used it).
But then, if they were only sending texts, migrating wouldn't be expensive.
Well, maybe next time you get somebody that implements a standard.
With that kind of behavior (not letting you speak to anybody, the blocking is understandable), it's clear you shouldn't keep their services. So, you have now an opportunity to do it right, and make the next move cheaper.