The Prime Minister said he along with French and German leaders had emphasised to Mr Zelensky their support for Ukraine and the "fundamental principle" that Kyiv should be in charge of its own destiny.
Following the call, Sir Keir told broadcasters: "We all want a just and lasting peace. That's what the president of America wants. That's what we all want.
"And so we need to work from where we are to that end.
"But the principle that Ukraine must determine its future under its sovereignty is a fundamental principle."
His call with Mr Zelensky, French President Emmanuel Macron and German Chancellor Friedrich Merz follows reports that the White House had pressed Ukraine to agree a plan that would see Kyiv surrender territory and limit the size of its army.
The deal is said to have been negotiated by Donald Trump's special envoy Steve Witkoff and Kremlin representative Kirill Dmitriev.
Following Friday's call, Mr Zelensky said he and the other leaders "value the efforts of the United States, President Trump, and his team" and were "co-ordinating closely to make sure that the principled stances are taken into account".
He said: "We are working on the document prepared by the American side. This must be a plan that ensures a real and dignified peace."
Sir Keir spoke to Mr Zelensky from South Africa, where it is thought he intends to use his attendance at the G20 summit to shore up support for Kyiv.
On the flight to Johannesburg on Thursday night, the Prime Minister told reporters: "My position has always been focused on a just and lasting peace. That's why we have done so much work on the Coalition of the Willing.
"But that is premised on the principle underpinning that, which is central to me, which is that the future of Ukraine must be determined by Ukraine and we must never lose sight of that principle underpinning the just and lasting peace that we all want to see."