Comment by Nextgrid

Out of curiosity, what do you actually need out of a VoIP/SMS provider beyond "send and receive SIP" and "send and receive SMS"?

I'll give you that SIP is quite complex to deal with (aka the existing tooling around it is shit - the least shit is SipSorcery in C# but requires quite a bit of low-level code to get anywhere), but SMS is trivial as it maps nicely to HTTP's request/response paradigm. Just write whatever logic you want in a PHP/etc script, drop it on shared hosting, and enjoy better uptime (your shared hosting provider doesn't have the budget to keep paying techbros to constantly mess with the system, so it will be more stable).