In order to resolve this issue, disable the Advertise G.722 Codec Cisco CallManager Enterpise Service Parameter, and reset the device pools:
This is further documented in Cisco bug ID CSCsi61965.
When the service parameter is enabled, it allows these TNP/third generation Cisco IP Phones to negotiate and use the G.722 codec when calls are within the same region. Because of a lack of support for the codec type, this causes interoperability problem symptoms, such as call disconnects, for these features:
Ad-hoc Conferencing
Call Park