Here are the settings that work on my Nokia E51 phone and should be similar on other Nokia phones.
Goto Menu-> Tools-> Settings-> Connection-> SIP Settings-> New Sip
profile  (add a new profile as below)

Profile name: Callwithus (or whatever you want)
Service profile: IETF
Default access point: your wifi access point
Public user name: 123456789@sip.callwithus.com (where the numbers
correspond to your callwithus user name)
use compression: No
Registration: Always on
Use security: No

Registrar server
Registrar server address: sip:sip.callwithus.com
Realm: sip.callwithus.com
Username:123456789 (callwithus user name)
Password: your password (callwithus 6 digit password)
Transport type: UDP
Port: 5060