The call time limit has nothing to do with your iPhone 4, though that phone is no longer supported by Apple. The call time length is dependent on where you are when making your calls and the phone service company you use. If you are not very close to a cell tower, or you are not making a Wifi call (I do not know if the iPhone 4 does Wifi calls though, as I never owned one) when you are making or on a call, or there are a lot of devices trying to use the same cell towers, many calls and other things are likely to get disconnect, as there is an overload on the cell tower happening. And, it might not be your phone dropping the calls. It could be whomever you are talking to that dropped the call.
There are other things that can disrupt mobile calls too, for example, solar flares. Look it up. It's a well documented problem, with no way to fix yet. Dead spots - where there is no continual cell connection, such as in or near large buildings, mountains, hills, etc., can also interfere with connections to cell phone towers.