I'm still not exactly sure of what you are asking... if you have the new plan with 16gb or more (not 18gb) of data, shared between 4 lines, you can add the "Safety Zone" feature at no extra charge. What that does is slow the data to a crawl once you reach your 16gb limit, but you are not charged for overages either. You do have to add the feature to your account. If you have less than that amount of data, you pay $5/month for the Safety Zone.
If you do not activate the Safety Zone, you will be charged overages if you go over your limit.
With any of the new plans, if you do not use your limit, what is left rolls over to the next month. It expires after that month, so the only carry over is what is left from your base allowance each month, and that allowance gets used first each month, regardless of how much carryover you have. It's better than no carry over, but not ideal if you are very close to your limit each month - you can easily end up with overages.
IF you have the 18 GB plan (XXL) on the Old Verizon Plan, you do not get the extras until you switch to the NEW Verizon Plan. You can do this online or on the phone with CS. On threw plan, you will either need to save $10 and go to 16 GB (XL) or increase by $10 and go yo the 24GB plan (XXL). In either case, once you switch you are eligible for the carryover data, Canada and Mexico calls and roaming. You also get safety mode to prevent overages, but you must turn it on in the phone app (Free option with XL and XXL) plans.
SOPJOU84, we want you to take advantage of every benefit available on our network. Did you have questions or concerns regarding our new plans? http://vz.to/1P9xrru
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