We have tested on the following phones with very good results:
Vimphone - supports ZRTP
You may have mixed results, this is because their are so many variables when dealing with VoIP.
For the best connection speeds. WI-FI and direct connections to your home router will always give you the fastest speeds and best voice quality.
You will always get mixed results with 3G/4G coverage.
Give Boogo a good home, you will get the best results on the latest model smart phones and tablets and sip devices.
You can also install soft phones on your PC and use Boogo, we have received great results using laptops (PCs) using WI-FI.
We are always trying to improve and can only do this with your help.