i want to use my old number in my contract

HI i just get an contract to MTN last week the lady i spoke to i tell her i want to use my old number to my contract and they send the phone with the sim card when i go to the nearest mtn store in Mountain Mill in Worcester they have the number to call when i call the number they say i suppose to call in 24hours ago now i have to pay R786 to activate my old number i asked the lady to go back to my convesation with the lady i spoke to becouse they say the convesation is recorded and the they dont have a good customer care they just want money bad service ever even where i call their customer cere which is 135 their line always busy say i cant speak to the service champion their line are always busy so their easy option is to pay