When I played with my sim, I kept noticing the phone in his inventory. I thought it was a bug so I either sold it or put it through the recycle machine every time he got the phone. The game kept replacing the phone when ever I got rid of it. I don't know why the game kept replacing it, since it did the same thing our sim's phones already do, when we click on the phone tab to the left of the calendar at the bottom of our screen.
So I assume if you need it the game will put it in your inventory on any sim that is used to continue playing the event. Though I never used it in the sim's inventory. When I needed to use the phone I clicked on the phone tab at the bottom of the screen and didn't use it from his inventory.