Most email services require a phone number or other email service to sign up if there is anything fishy about your connection/browser (and there can be hundreds of things, even just some strange browser extension installed).

And there are services where you can buy a real temporary 4G/5G number (non-VoIP) for a service for less than a dollar per use. But that does add up after a while if you’re making tens of hundreds of thousands of accounts.