Without being able to get the full picture of the network topology, I'd offer the following comments to get started here:
1) There is no limit on CUCM call duration that would cause calls to cut off at 5 minutes.
2) You need to fully vet the routing between all sites - particularly between Site 1 and Site 3 as if there is a pretty close 5 minute mark on disconnects, you could have some issue with flapping/reconvergence of links within the network. It's hard to sometimes track this down but I would definitely focus in a solid effort there.
3) What phones, what firmware, and what codec is used between Site 1 / Site 3, Site 2 / Site 3, and Site 1 / Site 2?
First of all thanks for your reply , second I have developed a troubleshooting plan to isolate the problem and I found that problems related to the IP range assigned to Site1 (once using any other range problem disappear).
so I will check the routing, the FWs between the sites and feed back.
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