Who is using my sim card?

Last year i was ****** , had no insurance on the handsets, so eventually had to open a new/second contract with Cell C. My installments from last year have been inconsistent, and i would like to know who is now using my sim card, as i never went to collect the sim card from my first contract. I would assume the amount would stay the same since i left the sim card with Cell C.My sms notifying me of this new amount was received on the 1st of April.