Sometimes my iPhone5 cannot make a call: it sounds about three times and then it disconnects. A sign "Users busy" appears. I have to touch "Call back" to continue calling. Sometimes after talking a few words, it suddenly disconnects and sounds "click".

Sometimes my iPhone 5 cannot make a call: it sounds about three times and then it disconnects. A sign "User busy" appears. I have to tap "Call back" to continue making a call and it does. Somtimes after I having talked a few words, it suddenly disconnects with tit-tit-tit sounds. And then it sounds "click" as when we lock the screen. I have to make another call. The phone can connect this second time. Does anyone have this same trouble? Please help me. Thanks.