I have tested any voip clients, and I can tell that the best is Groundwire. I have been using Groundwire over 2-3 years, and the best points are:
- It works smooth, without facing any issues such as freezing or stuck.
- Smooth hand off between GSM and WiFi networks.
- Push notifications works perfect and app does not use battery if you didn’t use it.
- Developer replies within a few minutes every time I sent him an email.