Octrotalk - unfortunately inadequate as admitted in the description. 2 second call delays, echos, only works on speakerphone bad interface.
Viper I assume you mean viber because I could not find Viper. They don't have the HD edition available for the WP8 system yet so you cannot do custom SIP configurations.
There are a few I tried that didn't work and looked dodgy and not user-friendly which led me to changing my VOIP passcode just in case they are farming data.
The best looks like MobileVOIP but unfortunately my provider is not on the list :-(
A generic open system is still hugely needed - I will contact MobileVOIP too and see if there is any request list for providers or anything.
Not a resolved subject. Also I would have thought Nokia would start to build something for their Lumias considering their old Symbian phones had it all built in and it seems like a step backwards....Perhaps a post up there is worth it too.