The chips are available from a few suppliers. They are a piggyback type chip. It's a regular 24-pin wide DIP with a coin battery atop it potted with epoxy into a module that is taller than a conventional DIP. Adtran has soldered them to the PC board, so to repair you will need to be comfortable with through-hole component level repair. Observe static precautions. I would strongly suggest replacing the bad chip with a socket and plugging the new chip into the socket. Some have had success with digging out the potting compound and replacing the sealed battery within the defective chip. Make sure to note the orientation of pin 1 of the chip. See video linked below.
New chips still available (and sockets): http://www.epower2go.biz/catalog/index.php?main_page=index&cPath=9
Video of repair of existing chip: https://www.youtube.com/watch?v=NdlSfqto_0o
You will need to take the units out of service to repair. There isn't a practical way to retrofit with a replaceable battery other than atop the old chip as shown in the video.