Call Forwarding is a pretty common telephone feature that allows calls made to one number to be forwarded to another specific number. You are essentially telling your carrier to redirect all incoming calls.

You must turn on Call Forwarding so that when someone calls your primary smartphone number you receive the call on your Light Phone. You will turn on/off Call Forwarding when you go light or return to your smartphone.

There are a variety of ways to turn on/off Call Forwarding and we are trying our best to get to a universal solution for all carriers & devices.

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Using Device System Preferences

One of the easiest ways to turn on Call Forwarding is through the smartphone settings.
Below are the instructions for how to do so on iOS & Android devices. 

Creating a 'Light' Contact

Call Forwarding is activated with carrier codes, which are short codes that you call to tell your carrier to do various things, like turn on or off Call Forwarding.

Because the codes are phone numbers that you dial, you can also save them as a contact and easily trigger call forwarding on or off by calling the appropriate number.

The most universal code to turn on Call Forwarding is:

ON:   **21*+ Light Phone SIM Card #
OFF:   ##21#

Please note, certain carriers might use a unique code, if this standard carrier code does not work we recommmend checking with your carrier to learn about call forwarding.

SIM Twinning / Cloning

A method not legal in the United States, but possible else where in the world, is to duplicate/clone your SIM card. Doing so will make sure the Light Phone always uses your same phone number for incoming and outgoing calls and might even be free to use depending on your plan with your carrier as it might use your existing plan's minutes. We do not have much more information than this, but you can speak with your current carrier about whether or not such a method is possible.