It’s not exactly the most elegant solution, but this Phone 5 Dual SIM Adapter Case gets the job done, allowing you to toggle between the SIM card inside your iPhone and an external one.
The case itself is the simple part – it’s just a translucent plastic case. More importantly, the set comes with some smaller parts you’ll want to be careful with. Included is a modified Nano SIM tray and a ribbon cable. You’ll have to pop out the Nano SIM tray on your iPhone 5, thread the ribbon cable through the new tray, place the Nano SIM on that tray, that slide the new tray into the slot. The ribbon cable will be left hanging out, and you’ll attach your extra SIM card to that. It’s not pretty, but if you want dual-SIM, there you go. Instead of just having the ribbon cable dangle all over the place, you can tuck it behind the phone, then put the case on the back to keep your second SIM in place.
There’s a helpful how-to video on the product page, but the process isn’t as complicated as it sounds. Once you have the whole apparatus set up, switching between numbers can be done through the iPhone’s SIM settings. Technically, this will also work with the iPhone 4 and 4S, but the plastic case won’t fit, so you’ll have that dangling ribbon problem. Seems like that wouldn’t be the best idea if you stuff your iPhone in a pocket or purse (so, just about everyone).