Terrible experience yet again

I called telkom in Feb to change my bank details they asked me to please make a cash payment towards the account as they cannot change it immediately which is what I did.my contract is a top up R99 a month 04th April I receive an my bill showing my account was paid and today I receive a bill for R500 please tell me why? My bank details were confirmed ,my account has to be debited for the previous month bill to be paid how am I charged R500 ? Where are these amounts from.Telkom by far has the most *********** staff compliment as they can't update anything I suggest you first look into my account check all the previous payments and relook at this invoice .simple maths will tell you R99 a month .I have changed these details in February what has your staff done with the Info ?