If your IP (and possible your browser) looks "suspicious" or has been used by other users before, you need to add additional information for registration on gitlab.com, which includes your mobile phone number and possibly credit card information.
Since it is not possible to contribute or even report issues on open source projects without doing so, I do not think any open source project should use this service until they change that.
Policies like that are almost entirely about minimizing fraud and harassment. It really sucks for people who don't have mobile phones that support authentication texts or whatever (since, even as you pointed out, the requirement is mostly a phone number) but it also drastically cuts down on fake/harassment accounts.