some reasons or cause???
The cause might come from MT, BTS, BSC, or NSS itself. for example MT already sent a REL connection but MSC still in the conversation(MSC not give an acknowledgement to BTS then MT level)… So by hook or crook you have to do an analysis either signalling tracing, netmonitor or any tracing facilities in side MSC or BSC itself.
to do analysis;
1. may be you can get some information from the subsribers
i) mobile type
ii) exact location if possible.
Then you can run/execute the problem itself.-to solve the ‘random’ problem.
2. Ask the vendor to do it.