I think I found it - I was looking at the device level earlier. Please tell me if I'm on the right track.
Incoming Calling Party National Number Prefix - MGCP: This parameter defines the number, up to 16 digits, that is prefixed to an incoming national number, and provides a means to help you identify national numbers, if necessary. This parameter allows you to prefix specified digits to the calling number of an inbound call on the basis of the Type of Number field in an inbound offered call. It also allows you to strip a specified number of leading (starting from the left-hand side) digits for the incoming calling party number before prefixing it. The digits to strip can be specified as x:y where x represents the prefix and y represents the number of digits to be stripped; the colon is required to act as a separator between the prefix and the number of digits to be stripped. For example, 91010:6 which means that the prefix is 91010 and the number of leading digits to strip is 6. In this example, a national call from 2145551234 would get updated to 910101234.The maximum number of leading digits that can be stripped is 24 and the maximum number of digits that can be prefixed is 16.
When the prefix in this parameter is applied to the incoming calling party number at the MGCP device level, Cisco Unified Communications Manager includes the prefix in the calling party number field for all additional actions pertaining to the call, such as supplementary services including call forwarding, call park, etc., voice messaging, and CDR data. Several additional service parameters provide prefix functionality on the IP Phone side and the incoming gateway side for H.323 and SIP. See the Clusterwide Parameters (Device - Phone) section and the related gateway-specific sections in the Service Parameter Configuration window for more information.
Maximum length: 19
Allowed values: The maximum number of leading digits that can be stripped is 24 and the maximum number of digits that can be prefixed is 16. A colon is required to act as a separator between the prefix and the number of digits to be stripped. For example, 91010:6 which means that the prefix is 91010 and the number of leading digits to strip is 6. In this example, a national call from 2145551234 could get updated to 910101234. Valid characters include (0-9,#,*,+,:)
