CELL C - SHOCKING SERVICE

Shocking service - after being a loyal customer for over 20 years! I cancelled my contract to terminate on 23 May yet the debit order went off on 31st May. I contacted the customer service line 4 x only to be cut off, telling me its the wrong dept, telling me to call back on the same number 135, which I was doing. Then when I finally got through the guy tells me I need to get my details correct on the app which I dont have or use and refused to help me or put me through to someone who could help me. He actually put the phone down on me and I am GOBSMACKED! Totally unacceptable and actually it is theft that they have taken the money off my account. Not sure what to do now.