Day is currently a partner with boutique business consultancy Sulis Ventures and was recently appointed to the trustee board of UNICEF.
He was chief of staff, brand and communications at Orange until the merger with T-Mobile in 2010 that created EE, which he left at the end of 2013.
Jackson and Arrowsmith left EE last month to launch Up having been at the company and its predecessor Orange for more than ten years.
The agency also announced it is advising Orange Innovation, the R&D arm of French telecoms giant Orange (formerly France Telecom), which owns a 50 per cent stake in EE.
