I don't know--I think you should ask Grandstream to build a fingerprint reader into its ATAs...

Seriously, a PIN is probably the best way to go, just don't call it a PIN. Call it a 'customer verification code' so that it's clearly differentiated from any kind of password (billing, control panel, etc) or voicemail PIN/password.

If you were to cave to those wanting you to have a security question/answer protection scheme, I'd recommend that you allow each user to create their own security question (only have one question). That way, the customer can decide which information to 'reveal' to Voipo and can more likely come up with something they'll remember.

"What is your favorite car?" Are you kidding me