To register a VoIP-Device other than the UM-cablebox is difficult and not supported via UM.
Nobody knows how long your configuration works, everytime its possible that unitymedia changes something.
The easiest way is really is to crete for every number one LAN-Phone at the Fritzbox and to connect asterisk to this fritzbox.
I use the same configuration without any problems.
Can you give more info for the configuration? Also anybody tested fax ?