Whaaa? Is that a new thing? I've never needed to supply my phone# to GitHub, and there's no # associated with my profile. Also, I agree with you in being a little hesitant to give it out and is precisely why I didn't go further than the sign-up page for coder.com.
Is it true? I don't recall github ever asking for a phone number when logging in
Maybe they mean Coder asks for the phone number when one logs in with Github.