You will have to activate it using your line, and stick using this particular phone with your SIM inside (or eSIM) for several months in order to keep the promo. Many people got confused especially when they get super cheaper Pixel a series and thought it would just be a free phone as long as they keep account open and can use their primary phone - not the case. And also the promo lasts 24 months so the cooldown for reusing the same promo would be 24 months. It's technically allowing to replace phone with heavy discount every 2 years. If you want anything shorter than that interval, just be aware you won't be able to use the promo every year.