SBCS would not support that configuration. If you had a smartnet agreement on your CUCM, TAC would support your configuration to get things working.
It would be an odd configuration because ISR's are added as MGCP gateway's so that CUCM can have control over the gateway's ports. It also pushes the SRST config and dial plan to the gateway. If you add the UC500 as a H323 device, you will need to configure and maintain the dial plan on CME and you would lose the PRI Backhaul failover capability, that comes with MGCP.
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