CALL DIVERTS WHEN BUSY

MTN is the only network in SA that does not allow call diverts to be disabled when busy. I have tried to disable the divert/forwarding when the number is busy via Samsung's call settings and I have also tried ##67# to no avail. The error message i receive when doing it via the phone's call settings is that the "feature is not supported by the operator". The ##67# option says "call forwarding when busy Erasure was successful" however when I phone from another number and cancel the call before the unanswered divert takes place, the call gets diverted to the voicemail service. Is there no way of disabling diverts when busy without cancelling/deleting the voicemail box? The call centre agent said i must activate call waiting and then try and cancel the divert, but this also did not work.