I signed up with Engin for my Nokia E51. They told me that Voxalot was the VoIP mediator for their service, but so far none of the settings or suggestions in here have allowed me to register.
They did send me their "BYO" settings and setting up with
this guide* I was able to register using those and make inbound and outbound calls, however I can't call my Dad's or my brother's mobiles - among other mobiles - but I can call some mobiles. Dad's on Telstra GSM and my brother is overseas in Singapore. The fact that I can register suggests to me that my routers (and their firewalls) aren't the issue.
The call quality sucks at my end but others tell me it sounds good at their end, so I was thinking maybe it's a codec thing. Reading on here that those who have connected to voxalot have been pleased with the service has made me want to get it connected to try and isolate the sound issue. I've also installed the Nokia SIP VoIP Settings tool but haven't touched anything in there 'cos I'm out of my depth fiddling with that. Awaiting guidance if need be.
My E51 is using firmware 200.34.36 - original Nokia phone with no external branding (eg, Telstra, Optus, Singtel, etc.)
* Mods please remove this link if it's a problem