Manufacturers description
These Dock Adapters enable your iPhone 3G or iPhone 3G S to connect to the iPod Universal Dock and other products that use the Universal Dock system. The adapters are also designed to enable sound to pass through during speakerphone calls.
Simply insert the iPhone 3G Dock Adapter into the Universal Dock, then plug in your iPhone 3G or iPhone 3G S. When you want to connect an iPod to the Universal Dock, pop out the iPhone 3G Dock Adapter and insert the one for your iPod.