He will depart at the end of the year to take up the position of senior product manager at the mobile network, where he will be in charge of the network's music content, working out of the company's entertainment division in Leeds.
He joined Happen in January this year along with Alex Sullivan to strengthen the venture. Both have been responsible for the day-to-day running of the agency. Before moving to Happen Kay was new business director at Fallon.
The pair's most high-profile work was the "Tate Tracks" campaign in September which focused on a series of established musicians and bands from the worlds of hip hop, garage, indie and dance writing and recording new tracks to accompany an art exhibition at the Tate Modern gallery.
The artists included The Chemical Brothers, ex-Blur guitarist Graham Coxon, The Long Blondes and Willy Mason.
Every month, visitors to the gallery have had a chance to hear a new unreleased track at listening posts situated in the gallery next to the work of art that inspired it.
Kay said: "I am taking up this position as throughout my year at Happen the most exciting projects have been those where I have been able to get closer to the content, like our Tate Tracks project, and the Orange opportunity allows me to get closer to content production and innovation."
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