Technically there should have been a site survey done before the access points were installed, and that would have provided you (or should have if the person doing it was worth their salt), the TX the access points should be operating within.
That being said, what power levels do those phones operate at? Assuming they are lets say 14dBm, then you would want your access points to be running around the same power level to ensure no near/far issues occur.
Phones can be very particular with how you deploy wireless, so if your doing a voice over wireless solution, the wireless design is usually catered to those phones first and foremost.
You'll want to ensure you have fast-roaming (or as close to it as you can get) enabled. How are the phones authenticating? What channels do the phones support, and what channels are you using on the access points?
What power level are the access points settling onto (assuming your using the autoRF feature)?