Last year we built out a network and decided that we would try going entirely wireless in the office. WIFI for all the computers and WIFI for the phones using VOIP over WIFI. We have the phones running over a Towerstream 10MB wireless circuit and our office running over a Time Warner Cable 50/5 circuit.
The short of it was that the Cisco 525G2 phones, which have built in WIFI, just sucked. Call quality was terrible and after many months people just stopped using them entirely and would do everything on their cell phones. I thought for a while that it was the Towerstream circuit as I saw some dropped packets and inconsistent latency, and had them out a bunch of times to resolve the issues, but the problem persisted, so then I turned to troubleshooting the WIFI.
In troubleshooting the problem I discovered a few things. When I scanned the WIFI channels using inSSIDer and found that there were no less than 25 2.4GHZ signals on all channels being picked up in my office (have to love dense urban areas). 5GHZ however was barely in use. Unfortunately the Cisco 525G2 phones only support 2.4GHZ so I was unable to see if WIFI was the issue or if interference was the issue.
I installed Ethernet to a bunch of the phones and they worked great. IN the process of installing ethernet, I freed up a WIFI bridge that supports 2.4GHZ and 5GHZ, so as an experiment I put the bridge on 5GHZ and hooked it up to the Ethernet port on one of the phones, and the call quality improved dramatically.
The takeaway is that VOIP over WIFI seems to be VERY susceptible to interference. 2.4GHZ was unusable for us, but 5GHZ seems to work on initial testing, but in our case it is far less congested so I can't say for sure whether it is a 2.4GHZ/5GHZ issue or a congestion issue. Wired however is by far the best so if considering VOIP, try to wire it, if you can't wire it, check your congestion with a tool such as inSSIDer and make sure that you have open channels (and not that this can change without notice if someone changes a setting on their WIFI). If you need to use 5GHZ, use a bridge, possible with a switch if you need to connect multiple phones.