It's more than a little likely that they already have your number. They have your email address, and chances are that more than a few of your friends have a con

It's more than a little likely that they already have your number. They have your email address, and chances are that more than a few of your friends have a contact in their google contacts that has that same address alongside your phone number. You could argue that they can't be certain, but aggregated across however many of your friends have those same details stored for you they can make some pretty safe assumptions.