Hi, we are full stack .NET developers, we know our ways real good around WEB APIs, we have several of our own services exposing Web API with OData + OAuth2.0 security which is the latest standar.
We have other services that sends SMS, but using PLIVO instead of TWILLIO, but I'm sure we can help.
In fact, I'm willing to do a proof of concept of whatever you need, without being awarded, so you wont have any risk
I'll show you the proof of concept tomorrow and then you can decide if award me or going with another provider
Thanks!
Felice