What are you talking about, a billionaire would not offer most of his money for a cellphone in 1920. There is no way you actually believe that.
Of course they would, it would be the most valueable item on the planet (asuming there is an internet surrounding it). You could charge millions for todays 65" OLED TVs in 1980. The price for polio vaccine in 1920? Well, no billionaire would risk any of their offspring to suffer from polio.
We literally have billionaires flying to space and longing for other planets today - way more useless than a smartphone.