It works great!

I have used it for a couple of weeks.
I set up my Voipon (UK) account and incoming and outgoing calls work perfect in both 3G or Wifi.
When I receive a call, if the app is closed, I will receive a Push Notification, allowing me to take the call. Even when I am in the street connected by 3G. THAT IS GREAT!
Support have answered promptly to me when requesting some information about the acc config. Some times, when 3G is not very good the quality of the voice decrease. (Its logic). The Developer needs to keep working on the UI (I think is very poor) and also the ring tones options. Other than that, for the moment the best Voip App I have tried.

HoyttGringo about iSip -VOIP Sip Phone