Does it do it on all accounts on your site, or only specific ones.
The HTML for that field is simple:
<input type=text name=phone value="PhoneValue" />
Where "PhoneValue" is the "phone" field returned from the database.
I'd verify that it's doing it for only certain accounts, and if so, it's likely they somehow entered their username in the phone field when they saved the form (or last time they updated their profile).
Haha I never gave this too much thought but for a lot of my users accounts in the phone field is says: "Bare" I just thought it meant Empty, maye its populating my account name. It has it for all the accounts that do not have a phone number entered.