Tell me which vendor you are working on then the problem may be specific and known to that network.
The case you specified can be because of two reasons.
1. Get a DT done and see if this happens after intra cell HO if so there might be mapping issue of that TRX. but your explaination shows that this is less probable.
2. Check if DTX_HR and AMR are enabled on that city. and confirm vendor. Issue might be with the decoding of SID frames there.
Do reply with Vendor, DTX and AMR information.