As you're on iPhones, if you haven't already, a good thing to turn off is the option for WiFi-Assist. It should be called Cell Data Assist, as it helps a weak WiFi signal, using your Cell data.
This might clamp down on possible data usage leakage for you.
I just got my plan changed. I am able to use the calling function but I am not able to use the data service that is mentioned in my plan. When I turn on my data in my mobile it goes to sign into network and then it comes like the data can't be used. What should I do?
In order for more people to provide better and faster answer...you should start a new thread. You'll get better response that way. Try to provide as much info on your account as possible. Better still...provide a screen shot of your plan details like "My Data & Add-ons".
@kameshwarasekar- most of the time, if your data is not working, and you have data in your plan, it is due to incorrect APN settings. Some phones will automatically configure the APN settings for the Public Mobile network, and others won't. You can check the following topic for more info about configuring your APN settings.