Tired of trying

I had a modem delivered by Telkom (Not sure why), I already have the same modem, I have tried unsuccessfully several times to return or have the modem collected but nothing happens. I have gone to a Telkom shop but they don't want anything to do with it because it did not come from them and now I am being charged for it. Would appreciate any advice from Telkom on how to solve this problem. Regards, Vanessa