Free | iTunes Link
First, over a secure Wi-Fi connection this app works flawlessly. However, over 3G and public Wi-Fi it has had issues with one-way audio. 3CXPhone is the best SIP app for connecting to a 3CX Phone System. It provides sound quality that matches the iPhone itself when it is using a strong Wi-Fi connection but can degrade quickly if the connection is weak, which is not surprising for VoIP. On a positive note, it is always able to connect. So, the Stun issues should be easy to correct. In the administration, there is a lack of ability to make granular changes to the set up, which may be problematic for some users and a plus for novices.
Overall: Interface A, Sound Quality A+, Connection B, Setup B+
About the Process
The review of these SIP based iPhone apps for using VoIP was far from scientific. The very unscientific testing was done in 2010 across 3 states (IL,IN,MI). It involved using the app over 3G, joining public Wi-Fi, and secure Wi-Fi connections. When connected, each app was used and rated by its positive and negative qualities. Apps have been chosen based upon positive reviews and ability to connect to any SIP Trunk.
Please read about safe Wi-Fi habits (Free Wi-Fi the World Over) before using these apps on open Wi-Fi.