You will need to set this up as a third party sip endpoint..
Here are a few links to get you going
http://www.netcraftsmen.com/sip-endpoints-in-cisco-cucm-x-lite-as-an-example/
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/7_1_2/ccmcfg/bccm-712-cm/b09sip3p.html
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