There was an issue in CME where previous calls were not ending correctly. CSCsl12443
If you reboot the phone, does the first call always work?
Would suggest to check the CME release notes.
7921 requires at least CME 4.1, which equates to the 12.4(11)XJ branch.
http://www.cisco.com/cgi-bin/tablebuild.pl/ip-key
Also would suggest to use the latest 7921 release (1.2.1), which is on CCO.
Ensure that "dot11 arp-cache optional" is enabled as well.
See the 7921G Deployment Guide for further info.
http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/7921g/6_0/english/deployment/guide/7921dply.pdf