Issue - Setting up a Polycom RealPresence Trio 8800 Voip over a Fritzbox 7490

Hello,

I'm having some trouble setting up a RealPresence Trio 8800 via Fritzbox 7490. I did set up Line1, the SIPserver information (IP and Port of the Fritzbox) and the login credentials. Looks good so far, but it seems the Trio 8800 is unable to register itself. Status for Line 1:

Line 1: Not Registered

*SIP: Server-1: *.*.*.* (Not Registered 0)

AOR: Trio8800@*.*.*.*

*.*.*.* = IP of the Fritzbox

I tried to punch in the register code I get from the Fritzbox (for example *123456 then green), but it doesn't seem to work. 

Any ideas? Does VoiP even work via Fritzbox with a Trio 8800?