TBWA picked up the account following a pitch, run out of Siemens' head office in Germany, against the roster agencies Mother and Wieden & Kennedy Amsterdam.
Work will be developed in local markets with the UK account estimated to be worth £5 million for TBWA/London.
The TBWA/London chief executive, Andrew McGuinness, said that the account did not pose a client conflict with the advertising business it holds for 3, the third-generation mobile phone network operator.
"Siemens and 3 are complementary businesses - one is a handset manufacturer and the other is a mobile network. For Siemens, we look forward to ensuring the UK launch is the success that it deserves to be."
The win puts Wieden & Kennedy Amsterdam's place on the roster under threat.
It has handled Siemens' existing global mobile phone account since the end of 2000.
Last August, W&K Amsterdam was dealt a blow after losing a head-to-head pitch against Mother and Naked for the £25 million task of promoting its new design-led mobile phone hardware, called Xelibri, which aims to make the brand more appealing to young people.
Mother, which also holds the UK account for Orange, launched a pan-European advertising campaign for Xelibri in May.
Despite its leadership of the mobile phone market in Germany, Siemens continues to trail behind the global pace setters Nokia, Motorola and Sony Ericsson.