I ran into this problem on 2 56s's with 48 key phones and short runs. One fix was with longer station cable runs.
The real fix is what PhoneSol mentioned. There was a bad run of phones. ESI TS came to my site with some "prototypes" and a new software load and fixed my issue. Have ESI run the S/N of the affected phones and get the latest software.
I know this is late posting, but better late than never.
