I didn't need to provide a phone number to OpenAI when I signed up for ChatGPT via OAuth with my Google account (which also has no device or phone number associated with it). But maybe I was just lucky enough to register before the requirement existed.
I did need to put some effort into configuring TOTP 2FA for the Google account without first giving Google a phone number, but I was able to do it by simulating a WebAuthn device with DevTools, which satisfied the pre-requisite of an existing hardware token or phone number method of 2FA that I needed to gain the ability to enable TOTP 2FA.
Google is one of the bigcorps that has required a phone number based on secret sauce for quite a few years now, I think they may have been one of the first to demand it. Back then, one of the factors was whether your IP address had been used for signups previously but there were and are probably more ingredients that go into the formula. Registering from a clean phone without any SIM would often work when the web registration form did not on that same IP address, but again, only once or twice, and the account iirc often asked for a phone number if you then use it outside of that phone.
You also can't keep signing up for a Google account with the same phone number. It'll say it has been used too often for a few days iirc and then back off again (not sure how often you can repeat that). Otherwise that could be a usable proxy for hiding the phone number from the target site, depending on whether Google gives that information to the third party you're signing up for.
Some of my old Google accounts also don't have a phone number attached, but using that for signups in various places would be an identity as stable as having a phone number.
Yeah, I haven't gone to great lengths to hide my identity. I just wanted an account for shitposting on YouTube comments. I'm sure the feds can find me if they don't like what I'm asking Chat GPT, but at least my conversations won't show up in the results of any JOIN queries for my phone number or email address.
I think in terms of privacy, you are worse off giving them a used Google account than a phone number. By using Google for this, you let Google know what you are logging in to OpenAI.
More generally I'm just getting tired of corps requiring phone numbers. If only there is something like SimpleLogin but for phone numbers (generating single purpose numbers for each source) to prevent phone number collection and spam.
I did need to put some effort into configuring TOTP 2FA for the Google account without first giving Google a phone number, but I was able to do it by simulating a WebAuthn device with DevTools, which satisfied the pre-requisite of an existing hardware token or phone number method of 2FA that I needed to gain the ability to enable TOTP 2FA.