Some older 4G devices might not be 4G calling compatible. You can check your device on our device support page. Pick your device, go to Network, then check that 4G Calling (VoLTE UK & Abroad) is ticked.
If your device is compatible, all you need to do is make sure 4G Calling is switched on in your device settings.
If it’s not compatible, you’ll still be able to send and receive texts and use 4G data temporarily. But you won’t be able to make or receive calls using 4G.
When we switch off 3G, calls, texts, and data will only be available when using a 4G Calling – VoLTE device. If your device isn’t compatible, you’ll need to change to one that’s compatible with 4G Calling to continue using our network.
You don't need to do anything if you have a 4G data-only device, such as a tablet. You can continue using our 4G network as usual.
Smarty runs on Three's network, doesn't it? Because iPhone 5c doesn't support 4G Calling on Three's network then I'm afraid you won't be able to make/receive calls as per above.