Account by different companies

Cell c decided to hand over my account to SSDA beginning of the year , I’ve been dealing with SSDA and paying them on a month to month basis and had no issues . Suddenly I’m receiving calls from Cell C collections to find out when will I pay my account , I’m abit confused because I am paying monthly to SSDA so why must I pay Cell C too ? Funny enough , now I’m getting calls from VVM saying my account has been handed over to them as well , I don’t understand why I need to pay all three companies when cell c initially handed my account over to SSDA , with no communication now I need to pay VVM ? This does not make sense the service has been appalling from cell c and I wish to not do further business with them , very slow and *********** service