Vodacom the money *** of cellphones

I pay my Vodacom account on the 16th of every month. The last sms was that R394.57 was outstanding, I paid R400 on the 16th of October. However, another sms state that my account is in arrears with R394.57. It's impossible as a Vodacom customer to pay up to date, because there is always an amount payable and I never know for which month.