NEOTEL POOR CUSTOMER SERVICE

I bought a home phone with you guys and had problems in loading airtime as the gadget is not taking the vouchers. and is not even going through when you call . I reported this long time ago , it has been more than 6 months now , I called several times and would be told they do not see anything wrong with the line and that I should go and try it again , only to experience the same problems. what should a customer do to get proper assistance from you guys? why would you sell to clients if you are lazy to assist when there are problems? can this be solved urgently or be returned back if you cant take responsibility in this please.