Hi David,
The ability to change this was added with the release
of firmware 9-2-2 I believe
Multiple Calls Per Line
The Cisco Unified IP Phones 8941 and 8945 now support Multiple Calls Per Line.
Each line can support multiple calls. By default, the Cisco Unified IP Phones 8941 and 8945 support four active calls per line, and a maximum of 24 active calls per line. Only one call can be connected at any time; other calls are automatically placed on hold.
The system allows you to configure the maximum calls/busy trigger to not more than 24/24 both for SCCP and SIP. The default call/busy trigger value is 4/2.
This feature is supported on both SIP and SCCP phones, with Unified CM 7.1(5) and later.
http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/8941_8945/firmware/9_2_2/english/release/notes/8941_8945_922.html#wp206145
Cheers!
Rob
"Why do the best things always disappear "
- The Band