The two shops were chosen after a trawl of about a dozen agencies around Europe in advance of an expected launch of the products later this year.
Industry sources say the groundwork, which has been shrouded in secrecy, is being carried out by a special unit within Siemens, operating at arm's length from the main company and investigating how to get young consumers to reappraise the Siemens brand.
The result will be a new range of products with an emphasis on style rather than technological wizardry.
The move is seen as a response to the fact that mobile phones have become a high volume but low margin business, whose future is dependent on new technology which is slow to arrive and for which consumer demand is unproven.
For Weiden & Kennedy, the assignment would complement the £70 million Siemens global mobile phone business, which it has handled since the end of 2000.
Mother has not previously worked in the telecoms sector but has a reputation for successfully advertising youth-oriented brands such as Dr Pepper and Kiss FM.
Despite its leadership of the mobile phone market in Germany, Siemens continues to trail behind the global pace setters Nokia, Motorola and Sony Ericsson.
Some telecoms marketers suggest Siemens could be launching the new range as a "loss leader in order to stimulate awareness of its name in the mobile sector. Its brand image is said to have been "blurred by its presence in so many areas from white goods to heavy engineering.
The drive toward stylish rather than hi-tech mobiles is being fuelled by increasingly low prices, which have turned the products into disposable items.
As a result, the market has plateaued and the next generation of technology has been slow to materialise. There are also doubts that technology-saturated consumers will be willing to pay premium rates to download video clips or for being able to send e-mails while on the move.
"Margins are tight because there's no added value in the market, an industry source said. "That's why persuading young people to buy the coolest-looking phones around suddenly becomes very important."