How can I record call in my iPhone 7s?

There is not a native way to record a call on the iPhone. Eavesdropping laws differ in many countries, as well as even in different states and cities. Because of that, Apple does not provide a method. You can look at apps, however understand that many of them have your call go through their servers to enable them to record the call, so they have complete access to all of your conversation. A big privacy violation in my book. Up to you to determine if you wish to use an app for that purpose and to determine which one to use.

You're welcome.