It picked up the business after a three-way pitch against JWT and Publicis.
The Engine Group companies involved in the shoot-out were WCRS (above the line), Meme (digital) and Dave (brand identity).
The agency's first brief will be to create a global business-to-business corporate campaign that will include TV, press, online and brand-building for the company.
The campaign will be the first from the newly created telecoms giant, as it prepares for its global launch in January at the 3GSM trade show to be held in Barcelona.
Nokia Siemens Networks formed in June when the two telecoms giants merged their phone network businesses.
Grey London handles the advertising duties on Nokia's range of mobile handsets, while McCann Erickson London handles Siemens' mobile phones, which are now branded BenQ.
The Engine Group refused to comment on the win.