The Prime Minister and the US president have discussed strategies for withdrawal from Afghanistan, with the Americans due to slash troop numbers below 10,000 within months.
Barack Obama tonight announced the significant cut in troop numbers by the start of next year, with a view to completing the US withdrawal by the end of his presidency.
British troops are due to be out of Afghanistan by the end of this year, except for a small force left in Kabul to train local troops.
In a statement, a Downing Street spokeswoman said: "The Prime Minister spoke to President Obama yesterday evening ahead of seeing each other at the G7 meeting in Brussels next week.
"The call focused on Afghanistan, following the president's visit there at the weekend and in the context of the end of Isaf combat operations at the end of this year.
"They discussed the latest situation on the ground, including the successful first round in the presidential election; the prospects of the US signing a bilateral security agreement with the next Afghan president; and the capability of Afghan forces. And the president set out his thinking on the US military commitment to Afghanistan from 2015 onwards."
British troops are working to leave Camp Bastion in Helmand province following operations there since 2006. Between 160 and 180 British service personnel will continue to run a training facility north-west of Kabul.
Mr Obama's strategy will see current troop levels drop from 32,000 to 9,800 at the start of next year. Including all Nato troops, around 12,000 foreign soldiers will be in Afghanistan at the start of 2015.
Speaking from the White House Rose Garden, Mr Obama said: "We have now been in Afghanistan longer than many Americans expected.
"Now we're finishing the job we've started."
Reflecting the fact the first American troops went into Afghanistan within a month of the September 11 attacks, the president added: "It's harder to end wars than to begin them."
During their call, Mr Cameron and Mr Obama also discussed the ongoing situations in Ukraine and Syria.
The spokeswoman said: " They also discussed the situation in Ukraine following yesterday's presidential election and agreed that the G7 must send a clear message of support to the Ukrainian government at next week's summit in Brussels.
"Finally, they agreed to have a more in-depth discussion on Syria next week in the margins of the G7 meeting, particularly how the UK and US can work together to tackle the ongoing threat to our national security."