How do you indicate the main or primary phone number for a resource?

I’m just curious how others are indicating (within spec) which phone number is the main phone number for a resource (service, location, etc.).

Are you enumerating description for this? Adding your own field to denote main or priority number? Cross-referencing schedule and displaying the phone that is currently active?

In one of our use cases, we need to indicate the main or default phone number that viewers should call, but list all phone numbers in details.