That's controlled with en_bacd_allagentsbusy.au, which should be in flash: and it is in the cme-b-acd-2.1.2.3.zip. Re-record that to what you want it to say in the same format (8k, 8-bit, mu-law, .au), upload and overwrite the old audio file, and reload the script/prompts (or reload the router) for the new file to be applied.