Just a quick tip...
if you're at the end of your data plan and you don't want to risk overcharges, you can still use your phone in Wi-Fi mode. It's easy:
With an iPhone, go to Settings and turn off Cellular Data.
With an Android, it is usually under Settings/More Networks/Mobile Networks. There should be a check box to turn mobile data off. There may also be a settings icon somewhere else, and every Android installation can be different.
The point is that you can completely isolate your phone from cellular data so there is no possible way you can go over. Unfortunately, this also isolates you from GPS and may also limit your ability to get caller ID or voicemail, so it is an option of last resort. You'll want to turn that cellular data back on as soon as your plan rolls over.