When I first got my Galaxy Nexus, I was one of the Verizon customers that was still "grandfathered" into unlimited data for $30/mo. I kept the Galaxy Nexus way past its shelf life because I knew if I upgraded to a new phone, per Verizon's new policy, I would have to upgrade my data to their tiered data plan or pay full price for the phone. The $30/mo option being just 2GB.
Finally in August 2014, months after my contract and upgrade eligibility has passed, my Nexus was beyond frustrating to use. I knew the phone I wanted was the Galaxy S5. I reviewed my data usage from August 2013 through July 2014 and saw that it would range from 1-1.4GB per month. I reluctantly decided to upgrade the phone and therefore had to select a new tiered data plan. Base on my data usage and not wanting my bill to increase, I selected the $30/mo 2GB data option.
From August-December 2014, my data was ranging from 2.1-2.7GB per month. Therefore Verizon was hitting me with a $10/mo overage fee. My daily usage had not increased. Like when I had my Nexus, I would always ensure I was on WiFi when at work and at home. I also ensured that I checked the option in settings that would keep WiFi on during sleep mode. I was baffled by my significant increase in data usage. I reviewed billing statements and was further confused on how I could have data usage during my workday and while I knew I was at home.
After months of research, I was able to identify that the Galaxy Nexus, when I was connected to WiFi, would completely disconnect from the mobile network. The Galaxy S5 and many newer phones do NOT do this. You will notice that the WiFi notification will be on WITH the 4G/3G notification. This was done in some lame excuse to "improve data speed " for the user and unbeknownst to the users. I know I certainly do not need "improved data speed" when using my blazing fast FiOS home broadband network or the like. Seemed like a clever way for Verizon (mind you AT&T does this too) to pull a fast one on its customers.
The only way to prevent this is to go to your settings and manually disconnect the mobile data EVERY time you are using WiFi. I have been doing this and now my data usage on my bill shows that there are no mobile data being used during my workday or while I'm at home. My best suggestion for making this easy is to include the mobile data on/off option as one of your quick setting buttons at the top of the notification panel.
I've heard if you contact your Verizon (or AT&T) rep, that they may credit your account for some data overages if you weren't aware of this practice. I've talked to at least two reps though and they are either lying or misinformed that this practice even exists.
Verizon coverage is superior to all other companies, especially for those not in big cities. However, I live in a major metro area. After over a dozen years as a Verizon wireless customer, I am considering going to T-Mobile, especially with their new Data Stash. Although for those in more rural areas that might be more difficult to leave Verizon, I would HIGHLY recommend using my solution. At a minimum, I hope this information will help you going forward.