Replacement 2256HS Cordless Handset?

The "7" key is sticking on one of our two cordless handsets that come with the AT&T 2256HS Cordless Phone System. Is it possible to fix it or more practically, get a replacemnt handset? And if replacement handsets are available, how can I make sure it will be on the same frequency as the "good handset? Are there dip switches inside to adjust the frequency? We can't find our instructions and I can't seem to find them on the Internet.