I am frequently asked by cell phone users who are experiencing cell phone reception & data connection problems. Why if I have 5 full bars of coverage do my calls go to voicemail? Why can't I access data on my phone or receive text messages? Its not because you are standing very close to a cell phone tower or antenna and getting a strong signal as most would think. Its because the cell phone network you are trying use is congested. Smartphones use ten times more data than cell phones that simply are used for making phone calls. Yet, a typical cell phone tower can handle up to 500 simultaneous phone calls it can only handle 1/10 of the users for data traffic. So if there are lot of iPhone users around you signaling to the tower it is likely that AT&T cannot handle the traffic. I am sad to say it is only going to get worse as more and more people start using smartphones that suck down the data.
My suggestion is pretty simple. Use WiFi whenever possible because 3G and 4G is not going to live up to the marketing hype.