If these devices are unregistering then they will log an event in the Application event log and CCM trace log. You can look for events related to your hosts and look for DeviceUnregistered or DeviceTransientConnection events. Look in the description information for the reason code. The reason code will give you a good starting place.
Of course this is assuming you are using CM3.x or 4.x. Not sure from your post which CM version you are using.
On the gateway itself, you can easily check if someone has rebooted by doing a 'show version' and looking at the uptime. You can also look at the 'show sccp statistics' to get an idea for how many times your gateway flaps.
Thus far, this information will confirm what RTMT is telling you and possibly (with the event logs) give you an idea if the device was reset manually. If the device is unregistering unexpectedly. I would check the local log and/or syslog host for your IOS gateway to see if you are getting Traceback errors or any other 'software' related problem message.
Of course, you will need to check the network path from the CallManager to the IOS gateway. Some questions to ask are if there is another gateway on the same logical segment or in the same physical path that can be used to validate the network. For that matter, you can look at the CM logs to see if you have multiple DeviceUnregistered or TransientConnection events at the same time your IOS gateways flap.
Without more information this is kind of 10,000 foot troubleshooting. Hope it helps anyway.
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