Telkom what a *******

So last year I took out a contract with telkom which was for a cellphone and a pocket router for 10GB for R139.00 a month for 24 months. I don't check the invoices they send since I know that it's a fixed amount. So yesterday checked that invoice out and to my surprise I found out that I am paying R139.00 for 500MB and not for the 10GB that I signed up for. Then I went and looked for previous invoices and yes I took a contract for 500MB a month. Telkom must have seen that I'm not the type that checks my invoices and used that to rob me.