I've also heard at least one late-night comic bring that up in the same ominous tones, so you're not alone. But why do you all assume it was a (voice-only) telephone? Aside from all the fictional portrayals of the President talking to other heads of state on elaborate videoconferencing setups (standard equipment in corporate conference rooms for decades), Trump insists on using his personal smartphone, which presumably has several video calling apps on it. Or his control at Moscow Center could have supplied the equipment.

I don't know one way or another whether Trump checks in with his boss via voice or video call. The transcript at whitehouse.gov doesn't say. To me it's probably not the most consequential of the POS's 10,000 lies and counting.