I ran into this few times and wonder how the ATA looses its codec list (I assume that it is indeed an issue with Codecs).
The user goes offhook and no dialtone is played but rather the fast-busy tone (the one you get if you are offhook for too long) and the fast-fast-busy tone that goes after ... so no dialtone in the first place.
Further to this, any INVITE sent while the unit is in this state has a SIP 488 returned with a "Media type not available".
The only way to return the unit into proper service is to restart it.
My units have only one preferred codec configured. I am not using the second and third preferred codec. I guess I could set them to use the same PCMU value but I wonder how the ATA would get into that state in the first place. If I could reproduce it, then I would be in better position to see if configuring the second/third codec would help.