Gyro won the business following a competitive pitch against unnamed agencies. Gyro will initially be working on delivering high-impact acquisition campaigns, via print and online activity, along with sales tools and channel marketing.
The agency will also be working on a number of strategic projects targeted at the UK's business market.
Katherine East, head of business campaign management at T-Mobile, said: "T-Mobile has a growing business base in the UK and has some strong activity planned for 2007 and beyond.
"The appointment of Gyro, which demonstrated great creative and strategic thinking in the pitch process, will ensure that we drive the T-Mobile business brand forward in this increasingly important market."
Richard Perry, chief strategy officer of Gyro International, said: "This is another tremendous account win for Gyro. T-Mobile already has nearly 17m customers in the UK and obviously has a fantastic brand presence. We are looking forward to helping T-Mobile to re-energise its B2B proposition and drive new customers to the brand."