Following the phone call, a Downing Street spokesman said:
The Prime Minister telephoned Arlene Foster earlier today to congratulate her on her appointment as First Minister of Northern Ireland.
The Prime Minister reflected on the milestone of the first female First Minister being appointed, as well as the ministerial experience and excellent working relationships which Mrs Foster will bring to the role.
Mrs Foster and the Prime Minister agreed on the importance of implementing the Fresh Start and Stormont House Agreements and the Executive being able to focus on the economic issues which affect the everyday lives of people in Northern Ireland.
The First Minister and the Prime Minister agreed to maintain a close working relationship and regular dialogue to ensure progress in building a more prosperous and shared future for the people of Northern Ireland.