A full version program for iPhone, by CounterPath Corporation.

A softphone is a kind of phone that does not make any physical connections and is thus immune to many of the problems that traditional phones experience. This makes them ideal for use in a mobile environment where you don’t need to be physically connected to the rest of the world. Softphones are therefore perfect for use in a mobile enterprise setting where you might need to make calls from a remote location. They are also ideal for use in a public location where you might need to communicate with a lot of people at the same time. 

Bria Enterprise is the best softphone client for iOS and is specifically designed to work with the CounterPath Bria softphone.