There are chances your phone itself may be the reason for poor connectivity. If the internal antenna has been damaged (due to drops or exposure to water), it may lead to dropped calls. Your phone may also limit connectivity if the battery lacks sufficient power.
Quite often, the simplest solution is to turn your phone off and on again to re-establish a network connection.
Have you purchased your phone from another mobile provider or a Parallel Importer?
If your phone has been purchased from a parallel importer, the fault may be in your specific handset when it was purchased from overseas by an importer. You could take it in for servicing or you could consider upgrading to the latest 4G phone with a great deal.