Ostrom is due to leave his postion as director of communications at The Number 118 118, the company that operates the service, later this spring.
Stating that he was looking at potential UK-based in-house roles, Ostrom told PRWeek: "I’ve been here more than ten years and it’s been an extremely interesting job, but I now feel it’s time to do something new and explore the opportunities that are out there."
Ostrom joined the company in 2003, soon after BT’s original 192 directory enquiries number was switched off amid moves to deregulate the market.
He previously handled communications around the launch of O2 after BT Cellnet was spun off from the BT Group.
A spokesman for The Number 118 118, which is owned by New York-based information company kgb, said: "Ostrom has led the company’s external communications since its launch just over ten years ago, securing and consolidating the market leadership for 118 118, as well as similar businesses subsequently launched across Europe.
"He has decided that it is time to seek new opportunity in the UK. The Number has started a search for a new PR director and William will be leaving his role in due course."