Founder and core maintainer here, I was surprised to see this post.

I like to address some of the questions.

1. I'm using twilio to forward text messages. At first glance I thought this could replace it but it doesn't look like it does sms - @xrd

You are correct.

Fonoster does not yet support SMS. However, SMS is within the project's scope and will be implemented later this year. Please see the roadmap here:

https://github.com/orgs/fonoster/projects/9

2. So I still can’t wrap my head around what happens when you go from a Twilio api call to the cell tower and ultimately the handset - @atonse

Some use-cases will require a VoIP Service Provider. Specifically, every time you send or receive a call from the telephone network (PSTN). Alternatively, you could use a SIP-to-GSM Gateway such as GoIP 8.

Fonoster will be particularly helpful when there is regional regulatory compliance or when a company wants to reduce reliance on traditional CPaaS Providers.

Note: A VoIP provider is not needed for internal calling (like an office setup) or browser calling.

3. Would Verizon give me an IP to connect to? Which protocol?

Most telephony providers have SIP interconnection of some sort but are usually for big customers or to communicate with lower-tier providers.

But as @bzxcvbn pointed out, you can find a local VoIP Provider relatively easily. I use a few for testing, like DIDLogic, CallCentric, and VoIP MS.