I have 3 phones on Verizon, and was on the 2GB/mo data. 20 days into the plan, I saw we were at 1.566 GB, and figured this was a good time to try to shift plans mid cycle. I bumped to 4GB. With 2GB=$60 and 4GB = $70, I figured, worst case, I'd have the full 4GB for the month, and pay the full $70.
Instead, each was prorated (no problem) but since for the first 2/3 of the month, I was over 2/3 the allowance, I was hit with the $15 anyway. Which is funny, since I ended the full month at 1.976, and did this to avoid the risk of going over.
Nice guy on the phone, reversed the $15. This is not a rant, not even if I had to pay the $15. Just sharing the method their system uses to calculate the usage and overage. I needed the 4GB for the upcoming mo, so changing early just cost me a few dollars. From the above, the mid-cycle bump in plan can still trip the $15/GB overage fee.