*******!!!

I prematurely terminated my contract with Vodacom and they send me the outstanding balances to go and pay on the 23rd December 2015 and I did just that, now today I get an email saying I owe them R517.00 and when I called the call centre (subscriber collections) they told me that what I paid was for terminations and the invoice for December still has to be paid, So if that was the case why didn't they include it on the invoice? and why I never got airtime for the month of December if I was invoiced for it? Then the lady says they will take legal action if I don't pay this amount...