Cisco Gateway Ring Back OCS R2 issue update, take two:

Back in June I reported on what Cisco claimed was a fix for Cisco ISR gateways for a ringback issue that people deploying OCS R2 were experiencing.Inbound calls traversing a Cisco ISR gateway to OCS users were not receiving ring back on the caller endpoint. This included Cisco IP Phones and PSTN users.

I was told a new IOS version that in the long run didn’t seem to do anything but what was really needed was a command on the dial peer that pointed at the mediation server for inbound calls to OCS. This is best used with the certified IOS version c2800nm-advipservicesk9-mz.124-24.T.bin for interoperability for OCS R2. This command makes the router generate ring tone locally rather than rely on early media from OCS.

no voice call send-alert

dial-peer voice x voip
tone ringback alert-no-pi

This is a pretty easy fix and also works for IP-IP GW scenarios. Thanks to Jared on the Technet forums for sticking with the issue and finding the solution.