A lot of VoIP/SMS providers exist, but compared to Twilio, they are just DIY API and SIP providers, which might be what we as developers want, but not what a business "needs".
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).
Alpha tested with lightbulbs but is now a clear strategy taught to MBAs
It was an open agreement to avoid misleading marketing that would have (presumably) caused a "race to the bottom".