Hi Ross,
We are not running CCM 5.1 but this is the setting we use to tweak what is diplayed for ougoing calls;
External Phone Number Mask
Specify the mask that is used to format caller ID information for external (outbound) calls that are made from the associated device.
â¢The mask can contain up to 24 characters. Valid characters specify 0 to 9, *, #, and X.
â¢Enter the literal digits that you want to appear in the caller ID information and use Xs to represent the directory number of the associated device.
â¢See the following examples:
If you specify a mask of 972813XXXX, an external call from extension 1234 displays a caller ID number of 9728131234 if the Use External Phone Number Mask option is checked on the route pattern that is used to make the external call.
If you specify a mask of all literal digits, such as 9728135000 to represent a main attendant number, that literal number (9728135000) displays as the caller ID for an external call from any associated device.
From this CCM 5.1 doc;
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/5_1_3/ccmcfg/b08uspha.html
Hope this helps!
Rob