pay X to have your ad appear here for Y time Someone once told me how they got a bunch of cheap/used phones and just left them all running an app that shows ad

pay X to have your ad appear here for Y time Someone once told me how they got a bunch of cheap/used phones and just left them all running an app that shows ads. They'd glance at the phones now and then to see if they needed to "interact" with them to keep the ads rolling.