Originally, the "Complete Stranger" was either somebody on the (now-closed) Backstage Pass tour - there's a point where the tour is walking through hallways past the soundstages, and if there happened to be a group looking into the Millionaire Studio when the contestant chose that lifeline, the phone would ring up there - or on Mickey Avenue near where the Fast Pass dispensers used to be, outside where the Fastest Finger/disabled waiting area is. When Disney stopped that tour attraction, the only phone was the one on Mickey Avenue. Now, with the attraction waning in popularity and the machines - and therefore the attendant CM - no longer outside the theater, the phone's been moved inside.