$5.99 | iTunes Link
Surprisingly, when other apps have had problems with audio, iSip often does not, which makes it a good backup to have. However, it often could not connect to the voice server when other apps could. Like 3CXPhone, it performs best over a secure Wi-Fi connection and slightly better results over 3G, but not guaranteed. Additionally, the app suffers from a mid-range call quality and overly complex interface. Although, they have released a new "Basic" version for $0.99 that may be worth keeping on hand.
Overall: Interface B, Sound Quality B, 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.