Loss of network Cell C

Experiencing problem with network, whereby the phone is completely blocked and have to be reset to receive coverage. This is going on for almost 1 year. Cell C asked to replace phones, new simm cards and restart the phones 100 of times. But traveled oversee with no problem with other networks. This happens up to 4 times a day without warning. Many complaints logged, but each time the network department come up with ferry tale answers. The latest ref number ********** 686, ********** 493. Please resolve.