Plumblink asking for personal information

Went to Plumblink on Louis Botha for few things. At the till I was asked for my mobile number. I refused to give them my number and the cashier said that he can not serve me. I left the store and phoned the supervisor Layton. He told me that they need my cell number for my protection , if anyone is found with Covid19 they will phone me and warn me. And he said that this is instruction from the government. No other shop ever asked me for my number and refusing to help me.