Rule #2: No secret diplomacy
Any form of diplomatic discussion between opponents must be posted in the game chat in a language that all opponents understand. Diplomacy includes, but is not limited to: proposing truces, negotiating alliances, and coordinating attacks. Secret diplomacy can be hard to prove, but if you suspect it you should leave the players in question appropriate ratings. If you feel certain about players engaging in secret diplomacy consider reporting them in the Cheating & Abuse Reports forum by posting a new topic.
If there are 2 players in the game who are coordinating over the phone, then yes, it is cheating because it is not posted in chat.
If, however, one player is getting advice from someone not in the game, that is not against the rules. It sucks, but it's poor gameplay, not a coordinated attack. If it becomes an abuse of the partnership, then it will become a problem, but if it is an occasional thing, and especially if it is announced in chat, then it's perhaps a case for a bad rating.
For example, account sitting is allowed, but strategic sitting is not.
The Society of the Cooks offers training, but one person taking over another's account to win or "power level" is abuse.
Hope that helps.
Twill
p.s. moved to question and answer.