The Username is build from your countrynumber (49 = Germany) plus your complete IP-Phonenumber like this: (
49PreNumberYourNumber (without '+ and the leading 0 for the pre number)
ex: 49123412345
The password must be set in the 1&1 control-center (login.1und1.de).
You can use the answer-machine, too.
Set the same number as your username and choose „RFC 3842“ for MWI Type. The mailbox server ist
sip.1und1.de
Remember to set the SIP-Port to 5070 and forward it from your router.