You should be able to if the phones are from the same network/phone company and based on the same technology. (Size for instance) Although you will need to add the serial number to the phone companies records so that the sim will co-operate with the towers and authenticate you. Once added, all you have to do is call in and let them know you are using the other phone and they will switch you in an instant. Then you call back to let them know you switched back. On the action of switching, call first if you are using the cell phone to make the change. You should not get disconnected from the network until the call is terminated. Then you switch the sims. Try to make a call. After you have successfully placed a call, you are now ready to receive a call. It may take a few minutes before you can make a call. It used to be that you could not receive a call until you place one. It may be different on your network.