I am not aware of any WiFi phones that talk SIP natively (that does not mean they do not exist).
There may be some software-based, SIP softphone clients for PDAs, (Symbol makes some Windows Mobile PDA/barcode scanners that might have some softphone capabilities www.symbol.com ). But with typical PDAs, I believe that you will be swapping batteries quite frequently, they are typically not very rugged, and this approach probably will not be worth pursuing.
As an alternative, you may want to look into if your SIP provider is able to support SIP trunking OR you might find out if their premise equipment that can accept a T1 from another PBX (or in the case of Cisco Call Manager, a voice T1 port on a Cisco 2800 series router).
You could run a small Cisco Call Manager Express installation on a 2800 series Cisco router that would talk to the 7920 wireless phones and then trunk over to your hosted service premise equipment via a SIP trunk or a T1 voice port on the router. If you already have a Cisco Call manager system (and you don't mind keeping the SmartNet going on it) you could still use Cisco Call Manager to control the WiFi phones and then trunk over to the hosted provider's premise equipment via SIP or a voice T1. You would still need to use a Cisco router as the T1 gateway and, possibly may require it to provide SIP gateway functionality if the hosting service's premise equipment cannot be a SIP gateway to the Cisco phone system.
You will need to have someone knowledgable on Cisco VoIP as well as some SIP knowledge to pull this off.
Even then, keep in mind that SIP is not necessarily out-of-the-box plug-and-play ready. Even if can work through any interoperability issues and you establish connectivity, the types of features you may end up with may be simply answering and placing calls with not much else.
(Your mileage may vary)
