Hacker News Reader

Whether you buy phone from them or not, you still have a phone number associated with your account (cell phone etc), just a way to look up your record.

Are you sure they're not asking for the phone number of the party responsible for the account as a means of locating your account?

At least for my naked AT&T DSL, there's a 'fake' phone number (0-xxx-xxx-xxxx) associated with the account. Usually, the automated prompt won't recognise that number, but human representatives are fine with it.