And that the number on it didn't change... And that it still works... and that signal still offers voice readouts of the verification code... and that nobody else has the same idea and doesn't run into your same payphone (since they're getting rare this is legit a concern)... or that Signal doesn't get a list of payphone numbers somehow and ban them (like these services do with SIP-based numbers)...

This has way more problems then getting a minute phone with cash that you can just keep topped up enough to keep service running.