He took the call as the President-elect , otherwise he could get caught in the Logan Act.
President elect is not an office despite Obamas cheesy logo. And the logan act is not violated by someone taking a congratulatory phone call. Nothing was negotiated.
The logan act does not prevent a citizen from merely interacting with any foreigner, including a foreign leader.
All that having been said, I think you may have the Logan Act wrong. It IIRC prohibits diplomatic activity by private citizens. Trump IS a private citizen until he is sworn in as POTUS. When he is POTUS, he will have every right to engage in diplomacy.
All this is inconsequential compared to the massacre of the Constitution by Obozo and his minions. Personally, I hope that Trump talks to as many foreign leaders as he can before and after Inauguration and hits the ground running and hits America's enemies harder than they can imagine starting on Day 1.
He seems to have a real instinct and talent for this sort of thing. He is also making reds' heads explode by dissing our wonderful friends in Beijing, the heirs of Chairman Mao-Tze-Tung.