It wont be possible with CUCM 7 but is possible with CUCM 8.5 & later. Please check with your Telco for UnScreen ANI service.
This service will allow a 800 number to appear in CallerID to customer, while allowing the Calling Number to screened by Telco to determine such things 911 and location sourcing the call.
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To do this Telco is using the SIP FROM header populated with an 800 number to appear as the CallerID, and using a SIP P-Asserted Indentity (P-PAID)header to hold the source number of the caller.
Unscreened by Telco means, The FROM header is NOT screened, however the PAID is screened by Telco before Telco will complete the call. When screening the PAID the Telco is looking for one of their assigned DIDs. Using an assigned DID in the PAID Telco will be able to determine location of the phone sourcing the call.
In order to add the 800 number in front the number the way would be to configure the external phone mask at the directory number configuration, due to there is not a cluster wide parameter to apply the change to all the phones.
In the case of how to populate the PAI header you need to configure a SIP Normalization Scripts, this supported from CUCM 8.5 and above.
Following guides would be of help..
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/8_5_1/ccmcfg/b06scrpt.html
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/sip_tn/8_5_1/1-overview.html
Hope it helps ?
Pls rate the post if it does.
GP.
