As mentioned, this is a data charge.
The "pay as you go" is a GoPhone plan for basic phones without a data plan. Rather than pay a fixed amount for a set amount of use, it charges only for usage.
A smart phone should not be activated on a basic phone plan, and this is why.
Turning off cellular data on the phone will not completely prevent mobile data charges. The only way to prevent all mobile data is to have customer service turn off data access on the line.