Yes, you need to turn off the mobile mobile, not the one in the car. If the networks detect a clone they will almost certainly shut the whole lot down, especially if they are coming from seperate IMEI coded phones, which they will be. frankly, I use a pay as you go in the car and divert all incoming calls to it if its just a toutine short journey, but I swap sim cards if I'm spending hours in the car. Getting the sim in and out of my Samsung T-100 is easy though. The car comes with 2 sim card card holders as standard and you can register 2 sims to the Connect system. Which I have used once. Its not much cop when you do.