The exit of the PR sponsorship and events director, who could not be reached for comment, triggers the potential for a review of the group's four PR and sponsorship agencies.
Business PR director and acting head of PR Jayne Hutchinson said Orange was hunting for Byrne's replacement and refused to rule out a review of Orange's retained PR consultancies.
Orange uses Cake for PR around its music and film sponsorship and Trimedia Communications to promote its involvement in literature.
Prodigy Communications has a consumer technology contract, while M&C Saatchi promotes Orange's mobile content deals.
Formerly head of group media relations, Byrne was promoted last year after the departure of director of corporate affairs Denise Lewis.
The subsequent scrapping of Lewis's role made Byrne Orange's most senior PRO, and was part of a restructuring process spurred by parent France Telecom's buyout of Orange's remaining share capital in 2003.