RadioShack began offering wireless phones and service from Cingular Wireless on January 1 in its over 5,000 stores, transforming its stores literally overnight.

As of Sunday, the full line of Cingular products are now available in all RadioShack company owned stores and over 500 RadioShack franchise stores. The 10-year agreement between RadioShack and Cingular was first announced back in August.

“We’ve been diligently planning for months to effectively and efficiently manage the addition of Cingular to our wireless family,” said David Edmondson, RadioShack’s president and chief executive officer. “We’re excited that our customers now have access to Cingular’s exciting new technologies, handsets and competitive wireless plans.”

To kick off the change, RadioShack employees stayed late on December 31 and worked to change products, branding and signage throughout the company’s stores, and has been training their employees to be ready for the change.

“While the overnight conversion of more than 5,000 stores may have seemed daunting for people outside our company, this isn’t the first time we’ve had to manage such a massive undertaking,” said Edmondson. “We attribute the successful transformation to many factors, including the senior leadership on the transition team, our home office employees who worked through every contingency possible, and our store employees who performed admirably during our busiest sales time of the year to make the transition seamless to the eyes of the customers. We also have the utmost admiration for our new partner, Cingular, who worked hand-in-hand with us every step of the way to allow us to hit the ground running on January 1.”

To celebrate the changeover, RadioShack stores will offer the Motorola RAZR phone and Bluetooth headset for $29.99 after instant and mail-in rebates with a two year contract. The special pricing on the RAZR, which retails for $399.99, is only available through January 7.