Incorrect Billing

i took a deal with Telkom for a home landline, which went well for a few months. after a few months they contacted me to say that i owe them R4000 for data use, they claim this was data that was used over 2 months. i explained that i never used any telkom data because i have my own router at home, how could a person even use R4000 data over 2 months ??? , to date my query is not resolved and i have not heard anything from Telkom since.