The 802.11e, WMM, and Cisco Compatible Extension specifications help balance and prevent the overloading of a cell with audio streams. CAC determines whether there is enough channel capacity to start a call; if not, the phone can scan for another channel https://www.cisco.com/c/en/us/td/docs/wireless/controller/8-1/Enterprise-Mobility-8-1-Design- Guide/Enterprise_Mobility_8-1_Deployment_Guide/Chapter-9.html