von diesem Posting, hat das funktioniert (beachte jeweils das "sip:" vor einigen Parametern!):

1) In Tools -> Settings -> Connection -> SIP settings ->Create thefollowing profile

Profile Name: derSIPprofilname
Service Profile: IETF
Default Access point: [your WLAN AP name]
Public user name: sip:deinusername@sip.voipbuster.com
Use Compression: No
Registration: "Always on"
Use Security: No
--
Proxy Server: sip:sip.voipbuster.com
Realm: sip.voipbuster.com
User Name: deinusername
Password: deinpasswort
Allow loose routing: Yes
Transport Type: Auto
Port: 5060
--
Registrar Server: sip:sip.voipbuster.com
Realm: sip.voipbuster.com
User Name: deinusername
Password: deinpasswort
Transport Type: Auto
Port: 5060

Registration: "Always on" and "When needed" is the switch to turn on and off SIP registration. So typically when I'm configuring I choose "When needed", and after I have done all the 3 steps, I come back here to change to "Always on" to switch on SIP registration.

2) In Tools -> Settings -> Connection -> Internet Tel. Settings -> Create a new profile for "derSIPprofilname", name it "irgendwas"
3) In Connectivity -> Internet tel. -> Preferred profile -> Choose "derSIPprofilname"

At this point you might need to do the previous mentioned step: change Registration to "Always on" in "SIP settings". The phone should now start to get WLAN association, with a fat WLAN icon on top right screen indicating success. Then it will take a little while to register to the voipbuster.com, after which a small Internet call icon will appear next to the fat WLAN on icon. Voila! Now you are registered and ready to make calls!