So I noticed that many Bitcoin sites utilize 2 Factor Verification for new registrations.  This requires you to provide your cell # and they send you a code which you type in to verify.

I'm always hesitant to provide my cell # because I want to maintain my anonymity as much as possible.  Does anyone know if the phone # data is stored for 2 Factor Verification?  Of course I know it is for 2 Factor Authentication but I'm specifically referring to 'Verification".