overcharge

I used to pay around $260 a month for a shared 20 gig data plan. in middle of the billing period of billing period October 23 - November 22 i changed plans to a unlimited plans, for the new plan I was told by the att agent (that changed the plan for me) that I would be charged $230 a month for 7 phone lines, 4 of them $35/month and 3 of them $30/month =$230.

but in what happened is, i was overcharged a few hundred dollars. For the billing period October 23 - November 22 I was charged $543! And for the next billing period November 23 - December 22 it says I will be charged $273!

There's a set price I was given for the unlimited plan either $30 or $35 per line.

if i changed my plan mid month to a cheaper plan then the bill should be less than the old plan or at least the same not a few hundred dollars more!