I've been billed for the closed account

Good day... about two weeks back I received an sms on my phone saying that I need to pay, but I closed the account already last year, 2015 beginning of December... When I went to Mr Price for enquiries they told me that I'm owing Mr Price Mobile because I have their sim card... At MRP Home in Greenacres they tried to get hold of MRP Mobile, but with no luck...Yes I did receive Mr Price sim card last year, few days later after receiving a sim card I received a phone call from MRP Mobile explaining how the sim card works, but I never activated the sim card until today... it's still attached to it's original card... now I don't understand how I'm billed for something I didn't apply for nor used...