How do I activate FaceTime on my iPhone 13 Pro Max?

You can Facetime others using your Apple ID too. This does not need any activation. Just enable FaceTime, that's it.

To activate iMessage and FaceTime using your Phone number

1. First, check to know whether you have an SMS plan enabled for at least one SMS*** with your carrier. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM to you
2. If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One international SMS*** Only)
3. If you can't turn on or sign in to iMessage or FaceTime - Apple ...

*** You may not have an SMS plan (Domestic & International) enabled by your carrier for the SIM/eSIM that you use on your device. Apple Inc. does not provide SMS services.