CUCM can either have a direct SIP trunk integration or use CUBE for integrating both the CUCM and Nortel PBX.
See some documents:
http://www.cisco.com/c/en/us/support/docs/voice-unified-communications/unified-communications-manager-callmanager/7207-meridianpbx-cm.html
http://www.cisco.com/c/en/us/solutions/collateral/enterprise/unified-border-element-cube/609362.pdf
http://www.cisco.com/c/en/us/solutions/collateral/enterprise/unified-border-element-cube/777115.pdf
The above documents are for the Normal CUCM not the hosted one but I doubt there will be any difference at the application level.
-Terry
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