Zoiper Desktop Soft Phones
Zoiper is a VoIP softphone that lets you make chat or make voice and video calls with your friends, family, colleagues and business partners.
Unlike other software like Skype or Viber, it is open and can be used with any VoIP provider or PBX. Allowing for much more flexibility and cheaper or better quality termination.
Zoiper Mobile Devices
Configuration instructions for Android and iOS