Next time this happens, look at your phone's signal... if it is 2 bars or less (depends on cell model, but this is generally true) then it is your lack of reception causing the problem. If you have good reception (3+ bars), you can pretty much rule that out. If your phone is less than 1yr old, it is under warranty. Send it in under warranty replacement 1-800-801-1101, assuming you have not voided the warranty by getting the phone wet, dropping it (a pretty hard fall), etc. If you get a replacement phone and are still having this same problem, you will need to contact tech support. (Has this been only a recent problem? could be cell (tower) problems....)
After hearing your problem, I would guess it is a reception problem in the area if others with different phone models are experiencing it as well. If others are not, I would deem it reception OR defective phone... but definitely look at the signal next time this happens as that is my #1 guess.