The two agencies are being appointed to work on Everything Everywhere's consumer, business-to-business and corporate work.
The brand was formed last year following the merger of Orange and T-Mobile.
GolinHarris had previously worked for Orange since 2006. The new brief is thought to include work for sub-brands Orange and T-Mobile.
The two agencies pitched together for the business and were shortlisted in March, alongside Nelson Bostock and The Red Consultancy.
One tech agency head said that GolinHarris/Weber Shandwick was 'perhaps the obvious choice, given Orange's heritage with GolinHarris, and Weber's scale'.
The PR boss added: 'By aligning the brands, it gives them some comfort in working with people they know, while offering an alternative brand to address the fact that T-Mobile and Orange co-exist and address different audiences.'
Industry sources suggested more agency appointments were in the pipeline but Everything Everywhere declined to comment. GolinHarris and Weber Shandwick also failed to respond to calls and emails. Sources said both agencies were under strict instructions from the client not to speak.
Everything Everywhere is the UK's largest telecoms company by number of customers, at about 28 million.
It is a 50-50 joint venture between Deutsche Telekom and France Telecom and was formed in 2010 through the merger of their respective T-Mobile UK and Orange UK businesses.
Last November, Everything Everywhere appointed Mat Sears as head of PR. Sears, who was head of PR at Orange, reports to brand comms director Stuart Jackson.
The latest PR news comes weeks after CEO Tom Alexander resigned for 'personal reasons'.