Hi, I have recently encountered a problem with my 4S whereby I dial a number either directly from my contacts or using the keypad and for some calls, the phone connects, rings once and then immediately drops the call. Sometimes it appears to try and connect and then immediately says "call ended." The crazy thing is it seems to be random and is not specific to land lines or mobile numbers. I'm sure I had something like this once on a 3GS but just cant remember how to resolve it. I have tried switching the phone off and then back on again, have cleaned the SIM card and have tried Reset Network Settings, none of which seem to work. Any advice would be greatly appreciated. Thanks.