Very interesting. I have found 5.8Ghz phones sometimes use 2.4Ghz frequencies, possibly for control plane signalling. Have you tried using a spectrum analyzer or the Cisco/Cognio Spectrum Analyzer solution (PCMCIA and software) to verify?
The signal strength diminishes with distance, so moving the phones is the easiest if not most practical thing to do, along with reducing the AP transmit power. The phones are frequency-hoppers so its not likely you can change their behavior. Who's the phone manufacturer?
