How to use new Facebook Portal to call iPhone X
You will need to install Facebook Messenger on your iPhone and iPad in order to receive video calls from the Facebook Portal device.

After you have installed the app and logged in on both devices, others will be able to call you by selecting your contact. I.e. others will not be selecting which specific device to call, but the call will show up on all your devices, and you can select which one to take it on.