Knowledge :-)
If you want I can help you. Just PM me.

I installed the justvoip-app, on my computer and tried to call myself:
- my voip phone rings immediatly
- the justvoip-app rings also immediatly
- my IOM phone rings 5 seconds later (acceptable to me)

I let the calls come in one a queue, I created a welcome message, and put the caller on music (MoH). So the caller thinks I don't pick up the phone immediatly, thats all.

have bought some credit from justvoip.

I have around 8 DID number that get ring on 2 locations on voip phones. Parallel I have these numbers forward to my IOM mobile. Instead on voicetrading I now setup the justvoip and an extension and calling the sip/myunsername@sip.justvoip.com

As you said it works very well. I have made some testcalls and it even seems to work faster than calling through voicetrading.
As said I also use the pbxes callback option so I can call cheap using my iom mobile. For calling out I will still use voicetrading as their rates seem to be stable.
For now justvoip has a lower rate 6 cent (incl vat) compared to 10 cents for voicetrading.
But as you know with betamax they can change their rates to a higher one. In that case I can switch back to voicetrading. But thanks for the tip It works great.