Require some kind of work from the user so automated mass account creation is non-trivial. Preventing that was the initial point of phone verification anyway.
Whether this is the classical CPU/GPU Proof-of-Work, or a manual thing such as solving CAPTCHAs, is up to your personal taste :)
In the specific example of ChatGPT, they would have an easy source of work at hand: Have users rate existing statements of ChatGPT as true/untrue. If someone tries to create an account and they rate too many as the opposite of their actual truth-value then they are a bot. If the same questions which pop up during account creation are then simultaneously fed as input using an existing account, then it's also a bot.
> Surely not credit card either. At least there are now a couple of proxies were one can use ChatCPT via VPN I guess.
How would that bypass the need for a phone number?