Voice Over Wireless LANs

Insufficient cell overlap—Most people recommend 20-percent cell overlap of your APs in the 2.4GHz band and 15-percent overlap in the 5GHz band. The overlap ensures that your phones have a high probability of being able to connect to an AP while they're roaming. Insufficient overlap can result in a phone being unable to connect to an AP, meaning that the user will experience a dropped call.

Low-power transmissions—Handheld devices transmit at low power levels to conserve battery power. This means that the device transmit power level is often lower than that of the AP, so the device can hear the AP but the AP can't hear the device. The solution to this issue is to reduce the power level of your AP to match the handset. Unfortunately, reducing the transmit power level of your APs can introduce holes in your network coverage. As a result, you might need to physically move some of your APs or deploy more APs.

Interference—I know that you said there were no sources of interference. However, interference can be intermittent and therefore difficult to find. Look at the areas where the problem is occurring, and use a spectrum analyzer to monitor the airways for an extended period to determine whether you can detect intermittent sources of interference.