I usually refrain to make comments about the work of my former coworkers at Cisco product testing, but this one is too much worth noting. From IOS 12.4(11)XJ4 release notes:
CSCsj03494 I/O Memory corruption crash with IP communicator and 7961 IP Phones
Symptom A 2811 series router may crash due to I/O memory corruption.
Conditions Router running CME 4.1 with 12.4.11.XJ3 image and using IP communicator and/or IP phones
Workaround Stop using IP phones or IP communicator.