The confirmation was made by the United States’ Press Secretary, Sean Spicer during a press. He revealed that Donald Trump placed a call to Buhari, South African President, Jacob Zuma and also the President of Tunisia, all in Africa.
However, it is still not clear why the United States Government did not post a read-out of the phone call on its official white house website whitehouse.gov where other calls he placed to other prominent world leaders are easily provided.
US Press Secretary, Sean Spicer
Buhari's aide, Femi Adesina had on Monday, February 13, 2017 indicated that the the President was scheduled to speak with Trump following news of the his health rumours.
“President Trump encouraged President Buhari to keep up the good work he is doing, and also commended him for the efforts made in rescuing 24 of the Chibok Girls and the strides being taken by the Nigerian military.” Adesina said in a statement.
Here is a video confirmation by Spicer, that Trump spoke with Buhari. Watch from 51:16-53:00 and listen to him speak.