SoftPhoneApp

Note: SoftPhoneApp for iOS is a free SIP-based softphone client that will only work in conjunction with a subscription to an approved hosted business-grade virtual PBX.
Compatible with Apple iPhone, iPad, and iPod touch and uses a Wi-Fi or cellular data network connection to make and receive voice and video calls, send messages and see user presence. SoftPhoneApp puts business-class communications tools in your pocket so no matter where you are in the world, you stay connected.
An Australian based push service ensures enhanced battery life.
Important VOIP over mobile/cellular data notice:
Some mobile network operators may prohibit or restrict the use of VoIP functionality over their network and may also impose additional fees. You agree to learn and abide by your cellular carrier’s network restriction. SoftPhoneApp or any of its resellers will not be held liable for any charges, fees or liability imposed by your carrier for use of VoIP over mobile/cellular data.