As a seller, I can look at the order details and see a buyers phone number and can add it to the package.

Most likely the PO inputted the number at the counter, or, if the seller has an AusPost business account, it's possible they can add the number when creating the label.

Actually, when I send parcels overseas, the PO often asks me if I want to add the buyers phone number.

When you get the parcel, just look and see.

Having said that, I generally don't click any links sent to me as an SMS, as I know that there are scams out there.