I did exactly this for a customer last year. I used non-gatekeeper controlled trunks and it worked without any problems.
CCM3.3 does not support Call Forward Unregistered though so you willhave to rethink that part (Forward All, Forward Busy and Forward No Answer are supported)
Create a route pattern of 2XXX (DN's were in the range 2000 to 2999) on CM3.3 pointing to the trunk to CUCM7.1
Create an isolation partition on CCM3.3 which was not in any CSS's
To migrate a CCM3.3 phone I would move its DN from Internal_PT to Isolation_PT which causes the dialled digits to match the route pattern created in step 1.
The migration went smoothly - it was interesting to be reminded how simple CCM3.3 was compared to recent versions of CUCM.
Another thing that I always suggest to customers is to ensure that the 'incoming CSS' of the ICT on each cluster doesn't have access to the partition of the Route Pattern to the ICT. This way, there is no chance of a call loop across the ICT between the clusters, preventing code yellow situations on the CCM servers.
That is good news. I remember having an issue with an ICT between 3.3 and an early version of 5.
Lets hope that was a bug just in that code.
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