When you use the application to make calls, Num2 creates a temporary contact on your device and deletes it at the end of each call. The error message you receive on your device is due to an operating system setting on some Xiaomi devices.
To allow the app to create and remove the temporary contact on your Xiaomi device, log in to Security Settings >> Permissions >> Permissions >> give permission to add and remove contacts to the Num2 app.
It is important to note that the Num2 application uses only the permission of contacts for the proper operation of the application. The app does not collect contact information from your device.