Are you able to provide more information about what is happening? Do you have a home network? Wired, wireless....?
Do you mean that someone accesses your Samsung TV via your home wireless network and launches your YouTube app?
Or that the TV just starts up and begins playing YouTube clips etc.?
Do you actually see the cell phone number? If so, do a reverse lookup and see if you can find out who has that number.
Would hope that the problem is just some configuration error or mix-up. Need more information to possibly help.