Here's how to use multiple SIM cards in a single iPhone

How do you install an extra SIM or two into the iPhone without jailbreaking?

Enter the SIMore X-Twin 6S Plus, a dual SIM device that allows you to have 2 SIM cards (1x nano SIM, and 1x standard SIM) in your smartphone. 

The nano SIM sits in the SIM card tray along with the tail of the X-Twin 6S Plus adapter so that it makes contact with the SIM card pin assembly inside the handset 

You switch between SIM cards from the iOS interface by going Settings >Phone > SIM Applications.

The kit includes the adapter and a case for your iPhone 6s and costs $40.

There are also versions available for other popular handsets, such as the Samsung Galaxy S5 and S6.