Wireless IP phones have never been popular mostly because a wireless access point is a "hub" style network device, and not a "switch". There's no dedicated bandwidth, and hand-off between access points (if your environment is too large for one to manage the area) can be frustratingly bad. Usually, a call will drop.
There have been improvements with wireless access points and phones, but you'd really have to dedicate a few access points to providing voice services, and not much else. Other wireless (laptops, etc.) network traffic is likely to degrade the audio.
Your best bet is to acquire a wireless phone that has a base station that is IP. Those have had better luck selling, though they're limited by the range of the phone and base station.
If you do decide to go the wireless IP route, you'd need a few wireless access points. Then, of course, the phones.
Carpe dialem! (Seize the line!)