Cell phone *****

Between 4 and 5 June I upgraded my cell phone at MTN Jeffreys Bay.The staff were very good.However something went wrong with my sim card which resulted in me having to buy a new one.On the evening of the 5 th at about 20h00 hours I received a call from a man who said He was from MTN ***** department and that someone is trying to access my account to do something *******.He proceeded to send me various OTP which I gave him. As it turned out He was the one to go into my account and de*****ed me of R 17500. My question is how did he manage to get my cell number and that at the same time as my upgrade ?He also phoned me at 01h10 on the morning of the 6th but I did not respond.