But just how Anglophile is he really, some ponder, given his mantra of “America First”?
Sir Keir had the conversation on his mobile in his office next to the cabinet room in No 10.
Team Trump rang the prime minister, after the government had requested a call with them to send their congratulations.
I’m told Trump had spoken to some other leaders first, but seemingly not many.
The plea from some in government, to themselves and an audience beyond Westminster, is to judge Trump by his actions, not by his words.
The verbal fireworks seem inevitable: that is the Trump way but don’t get distracted by them, is the mantra for some.
Not least because brash controversy and wild unpredictability is just the start of it. There is policy to think about too.
Take Ukraine.
If the soon-to-be president starts cutting support for Kyiv, how does Europe respond?
Does it remain broadly united or start to splinter?
If he demands again as he often has that Europe pays more for its defence, does it?
Can the British government afford to crank up defence spending more quickly? Can it afford not to?
Then there is climate change – and then the crucial issue of trade.
The president-elect has talked up the prospect of huge tariffs or import taxes on goods being brought into the United States.
What might this mean for Sir Keir Starmer’s central mission to try to catalyse economic growth?
If the European Union responds with retaliatory measures of its own, how should the UK respond?
Hug Europe close, or use the flexibilities of Brexit to choose a different approach?
Those who observed the first Trump administration closely tell me his significant mandate this time, and the wider Republican victories, mean the next president will be less restrained than last time.
They are better prepared to ensure they get their own people into the right jobs to get done what they want to get done and more quickly.
The implications, choices, trade-offs and dilemmas for the UK provoked by what has just happened in America are legion.