I had the same problem, but solved the problem differently.
I removed restrictions on accounts and location, on both iPad and iPhone.
Settings -> General -> Restrictions: accounts and location (both set to Allow Changes)
- Remove restrictions on Accounts and Location (set to Allow Changes)
- Turn off FaceTime on both devices.
- Turn off both devices and restart both.
- Turn on FaceTime on iPhone using only my phone number, without Apple ID. => Activation Ok
- Turn on FaceTime on iPad using Apple ID and default email address. => Activation Ok
- Try to call each other using other registered Number/email.
.... And worked ok for me.
I put back the restrictions On, on accounts and location, and it worked also ok afterwards.
There must be a bug somewhere related to these restrictions or something in that area. Apple should provide a fix to this. Restrictions also caused problem the message "Waiting for activation" and was solved in about the same manner as above.
Waiting for ios5.1 !......