Mother and Naked paired up to defeat Wieden & Kennedy Amsterdam in a head-to-head pitch held in Munich last Friday.
The news will be a bitter disappointment to Wieden & Kennedy, which has been the incumbent on the £70 million global phone business since the end of last year.
Although Mother has never worked on a client in the telecoms sector it has acquired a reputation for successfully advertising youth-oriented brands.
Equally, Naked has won plaudits for its work on PlayStation2 and Reebok and has experience of the telecoms sector through its work with Hutchison.
Stef Calcraft of Mother and Naked's Jon Wilkins led the pitch in what has has become a successful collaboration between the two agencies.
Details of the brand-led youth-oriented phone are being kept secret, but it will launch in the UK, Europe and China next spring.
The phone relies on GPRS technology and, following its initial launch, will be rolled out throughout the world, as the technology becomes available.
The first tranche of advertising has a budget in excess of £25 million in Europe.
The phone is expected to have an emphasis on style rather than technological specifications as the mobile phone market has become saturated as the hardware has got cheaper.
Siemens has also suffered from its image as the manufacturer of a wide range of consumer and industrial goods, which has meant that it may not necessarily be the brand of choice for a young consumer.
Siemens has yet to appoint an implementational media planning and buying agency for the brief.
Calcraft promised the communications strategy and marketing will be "different to the traditional prosaic approach". He added: "Siemens made it clear from the outset that it wanted new ideas and a new approach for this product.
"It asked us to do what we do best, encouraged us and partnered us throughout the pitching process and it was our turn to return the compliment last Friday. This is a stunning opportunity for all the companies involved."
Wilkins added the communications strategy that Naked had offered Siemens included all marketing elements.