Hidden costs/No assistance

I took a date line with Cell C in November 2018, the sim was never inserted in to a mobile device as it came with a modem. From Jan 2019 the monthly fee increase monthly, when queried I was told that it was because of services activated on the line. No services was activated as the sim was always used in the modem, how is it possible that services miraculously appear on the line. From Jan 2019 to date an additional amount of approximately R660.00 was deducted from my account.