iSip (formerly sipphone) from VNET is a SIP client for the Apple iPhone. It can be downloaded from the app store.
REQUIREMENTS:
You must have Callmanager version 5 or greater. I’m using Callmanager 6.1(2) for this howto.
Your iPhone must be able to connect to your Callmanager server – I’ve successfully used the iSip client via wifi as well [...]