Customer for more than 15 years, new 0800 number a...

Customer for more than 15 years, new 0800 number activated.
I get the invoice of the old 0800 and the new invoice 0800 number.
I am surprised to see that I pay half the price for the new 0800 number less than the old one.
Called about this I get to hear that this invoice can no longer be adjusted, it would have been adjusted for the next invoice {NOT THUS} PLEASE STILL DOUBLE THAN THE NEW 0800NUMBER.
Don't know how long I've been paying too much with my old 0800 number.