Using iPhone on WiFi (only) to run SIP Voice Applications

Hello all. Wondering if anyone else out there is trying to use the iPhone to host SIP-based voice clients. Specifically, to use the iPhone as a WiFi only SIP voice device.
We have been working on this idea for a while now at our hospital. We are replacing some antiquated DECT phones. We could just go with a newer VoIP phone set but there is a great interest in everything the iPhone can do on top of just voice. We are also looking at messaging (not SMS), reference apps, etc.

We are testing the unlocked (no SIM) iPhone 4S 16GB for hardware. We have tested the Avaya One-X SIP and CIsco Jabber clients with mixed results.
We will start testing the Vocera Connect app in a couple of weeks.
We have also talked to the folks at Voalte. It seems that they are laser focused on this solution (specifically for health care), but they are a little out of our price range.

We have also struggled to get out Cisco WLAN environment up to snuff to properly support voice. There is a big challenge making things right for the iPhone's WiFi since you will get zero help from Apple.

Would love to hear if anyone else is trying something similar and what experienced or advice you can provide.

Cheers!
Jeff