Overcharged for my phone subscription, how can I postpone my payment?

I dont know why I’m being overcharged for more than 100 euros, when I shouldnt. For a single phone line. I try to postpone the payment to be able to talk with customer service (hopefully going to a store because in the phone they don’t fix anything), and it doesn’t let me. Anyway, IF I should pay this amount of money, I should have around 150 euros credit from a lastpayment where I was overcharged and KPN itself gave, but it seems that credit disapeared. How can I postpone the payment and get my credit back without being 1000 hours in the phone???