From what you are describing, I believe either the Firmware on the ATLAS chassis or the Firmware on the module needs to be upgraded. You can find what FW is on the chassis by looking under SYSTEM INFO and then looking at the FIRMWARE REVISION. The latest FW is C0904 for the ATLAS 550.
For the FXS module you can look under MODULES and then go into the MENU for the FXS module, into INFO. The FIRMWARE REVISION for the module is listed under INFO. The latest FW for the FXS module is C04.
You can find and download Firmware for ADTRAN products here: Software Downloads For the ATLAS products this is either a ZIP file or a self-extracting ZIP (executable). Once extracted it includes the Firmware file (a BIN or BIZ file) as well as instructions on how to do the Firmware upgrade.
Hope this helps,
Since there hasn't been any follow-up questions I assume this answered your question - so I will mark it as "assumed answered."