Create a subrule to “Briefing”: “Contacts”, with the text:

An Agent may, if they have not yet done so when they had their current Location, telephone D-Ops requesting a contact. D-Ops should respond to this email.
Responses by D-Ops must be determined in the following manner:
If there are Agents or Impostors with a different Allegiance than the Agent requesting the contact, with the same Location as them, then there is a one in four chance that the response is one of their codenames.
Otherwise, the response is the codename of an Agent or Impostor with the same Allegiance and the same Location as the Agent requesting the contact, or “None” if no such Agent or Impostor exists.