Wrongly charged

My mom is a pensioner and cancelled her contract last year in March. She has gone over to Cell C since and she is still being charged but does not even have a vodacom number anymore. Everytime she goes to the vodacom shop they tell her it is cancelled but still she gets charged. They now again said she must phone the call centre but she never seems to get through. What must a person do to get them to cancel once and for all and to get my moms money back.