I've been using Vitelity (paid) but Twilio is a bit cheaper and has a better API. However, the more obnoxious confirmation code senders can detect all of these as being in data centers. IME it's only a few senders that are snotty about that. You could always get a burner phone.

Hmm, I don't know what happens if you get a mobile burner phone, set up call forwarding to your VOIP number, then throw the burner phone away (i.e. shut it off so you don't have to keep it powered and broadcasting its location). The cheapest mobile plan that I know of ($30/year redpocket) unfortunately went up to $45 a few months ago, but it gets you a usable backup sim.

Added: 1) r/nocontract on reddit showed a $36/year infimobile plan with a 20% off coupon (so a little under $30/y) on amazon. Similar deal to redpocket I think. 2) Another idea: get cheap mobile plan, port number into a voip provider, cancel mobile plan. I wonder if the number then reports as data center terminated.

There are now starting to be a few "free" mobile providers where you are required to keep a spyware app running. I don't think I'd bother with those. textnow.com is the one I remember but there were others. textnow does NOT support call forwarding on free plans.