BGR has learned that RadioShack will be offering a limited-time sale on all AT&T iPhone 4S and iPhone 4 models beginning Sunday, December 11th. The nationwide retailer will discount each iPhone 4 and 4S model by $29, and the iPhone 3GS will remain free on contract. AT&T’s iPhone 4 8GB and its older iPhone 4 16GB models will each be offered for $69.99 on contract, down from $99, and the carrier’s 32GB model will cost $169.99. BGR has confirmed with multiple RadioShack locations that both older iPhone 4 models are still in stock. Apple’s new iPhone 4S will be available starting at $169.99 for the 16GB AT&T model. The iPhone 4S 32GB will cost $269.99 during the sale and the 64GB model will be discounted to $369.99. All sale pricing requires a new 2-year service contract. It is unclear how long RadioShack’s sale on all AT&T iPhone models will run, and a company spokesperson did not immediately respond to a request for comment.