Whenever I pick up my iPhone, the cellular data service doesn't work. Any attempt to connect to the internet just returns an error message. However, this problem can be fixed by one of the following actions: 1. Toggling airplane mode on and off. 2. Removing and re-inserting the SIM card. Whenever I perform one of the above actions, cellular data will work for some time and then stop working after some time. What could be the problem? How could I fix this? This problem only began a few days ago (prior to which everything always worked perfectly on my iPhone). I'm currently in Europe using a prepaid SIM card with my factory unlocked iPhone 4. The iPhone is also jailbroken - but I don't know how that affects anything because the phone has been jailbroken for months whereas this problem only arose a few days ago.