Unknown — scam or not

Good day ,,,my name is Basie ,,,,I've recieved a msg from a certain Mr Williams with tel number +[protected] telling me that I've won 500,000 rand in a Nokia Promo ,,,

He told me to purchase a pre-paid phone voucher to the value of 275 Rand an phone him to connect my ref. number so that they can transfer the money into my account ,,,is this a scam or not and do I have to change my banking details