The brief is thought to be worth more than £100,000.
Text 100, which had held the account for three years, was not invited to take part in last December's pitch.
The contest is believed to have come down to a two-way shootout with Golin Harris.
B-M corporate and technology managing director Jonathan Jordan and director Jonathan Dinkeldein lead the account and will focus on Europe, the US and Japan.
Majority-owned by mobile giant Nokia, Symbian was set up to compete with Microsoft for dominance of operating systems for handsets, with advanced features such as video telephony and email.