I want a "burner" number, but I'm not sure what the best option is, do I buy a crappy phone at Walmart and use that number? What's the bare bottom of the barrel cost for a phone with no mobile data, only SMS?

I have an Ultra Mobile eSIM as a second line on my iPhone. Costs $3/mo and you can buy cards to top it up or add a credit card. My primary eSIM is a regular phone plan.

It works surprisingly well. I can easily turn off the second line in Settings without removing it.

I could’ve bought an unlocked phone with cash somewhere and used the SIM in that. They wouldn’t know who I was.

I didn’t do that because it was inconvenient and it wouldn’t be anonymous once I started using it for SMS authentication.

Wait, how did you get it so cheap?

TracFone with minutes at Walmart

If you load it with 10 dollars in minutes, it'll be good forever. But I'm not sure what TracFone has for an inactive policy

Supposedly as long as you keep paying for the plan, the minutes you load into it do not expire...

But tracfones don't have a plan. It's prepaid minute cards only.

Any VOIP provider with SMS support should do the job. I personally use voip.ms but there are many.