fields: a comma-separated list of the fields to return. Leave it out for all of them. Reach into a list with a dot:
contacts) returns all of its fields. account_id and signal_id always come back, so a row can be matched to the next call. An unknown field is a 422. On /analytics metrics, fields picks metrics.
Contact emails and phone numbers are opt-in: add include=contact_info. They are the details your workspace has already enriched; empty means not enriched yet.