3rd party APs including Linksys are not officially supported.
When in idle, will use U-APSD or PS-POLL. The 7921 also uses beacons for BSS status.
Not sure if that AP model supports WMM or not, but may want to try to disable it and also set on call power save to none, where the phone will be in active mode. Talk time will be reduced when in active mode.
I disable it, while reading Design Principles for WLAN:
"At the edge of each voice cell, the RSSI measurement should -67 dBm. For a Cisco 7921 voice deployment, it is recommended that you have a Received Signal Strength Indication (RSSI) above 35 at the edge of the cell, which is equivalent to -67dBm."