Two scenarios are possible,
1. A Faulty handset.
2. RF UL Quality issues causing no reception at the MSC Level.
Probably you should check if the issue is being generated from one IMEI or IMEIs of same vendor if its a faulty handset or a chinees company’s phone then its best to contact the customer and ask him to change his handset. Else you can improce the RF Quality of try increasing the T313 timmer.