Telkom is cheating me out of airtime

The following has happened to me too often to be a coincidence: I buy R65.00 airtime, then buy a telkom mobile voice bundle for R60.50 (200 minutes), leaving me with an airtime balance of R4.50. After confirming that the bundle has been registered, I then make a call to another telkom mobile number. After a few minutes I get disconnected and a balance enquiry shows that my airtime has been used up while my voice bundle remains at 200 minutes. ONLY after having consumed the remaining airtime will telkom start deducting from my voice bundle. I consider this practice to be ********* to put it mildly; more accurately I consider it to be downright dishonest and theft of my money. What can be done about it?