It's definitely not your phone causing the issue, many of us are experiencing this no matter how old or new the phone is, it's certain areas/postcodes getting affected due to masts configuration, I was told this by customer services but they will still try to tell you it's your phone settings. If you put another sim card in your phone, it will work and make/recieve calls. Three has done something with the network performance but won't admit it is affecting a lot of customers in a negative way. They expect us to change phones or provider
