Twilio works great. There are some caveats that some SIP Trunk providers offer that Twilio doesn’t.
Twilio doesn’t support T.38 for faxing, cannot update carrier CNAM, can’t purchase international DIDs everywhere (although they do offer several major countries), carrier mix is mostly Level3 and Bandwidth behind the scenes (not necessarily a bad thing, but Level3 has has issues time to time), free support is via email only with 24 hour response time (for non critical tickets).
If you are ok with that, then they are great. Stable, flat pricing, as many channels of voice as you need, e911, easy LNP port in and port out, call recording from the carrier (which is nice if you are trying to determine if an endpoint or PBX is having issues), trace logs (wireshark) for diags, etc.
They aren’t the least expensive for North America, and their international rates aren’t the cheapest either… but it’s predictable, lower cost than most other SIP providers, and their are no minimums… when i was introduced to them I just signed up for a trial, put a few dollars in for credits and I was off to the races.
They also offer other programmable services (video, SMS, cellular).