What is Smart Dual SIM on my Nokia 220/222/225 phone?

You can use Smart Dual SIM to forward your calls between your SIMs. When someone calls you on one SIM while you’re in a call on the other, you can manage your calls from both SIMs just as you would with only one SIM.

Make sure you have 2 SIM cards inserted in your phone. Select Menu > Smart Dual SIM, and change the settings depending on how you want to use the app.