Unhappy disappointed client

Had a contract with Telkom for data Sim for 22 months, all this time I've been paying my subscription of R249 monthly, April they said I owe R749, because on the 31st March I used R500 out of bundle rate in 1 day, but my argument is, first month I received the Sim, I asked them to lock it so that I can't connect when I deplete my data, now all of a sudden in one day the line is open and I use R500, I asked them on the 17th April to lodge a dispute for me at Telkom Baywest store, no one got back to me until today regarding my dispute