Were you able to determine any type of pattern when calls go to busy? The only thing I can recommend to help troubleshoot this given that the issue is intermittent is to enable some traces to monitor this passively. When the issue occurs stop the traces and track back in the logs to find out what happened to the particular calls that went busy. You would need to keep a good record of the calling/called number and time when the call was placed/received. If you open a case w/ TAC they would be able to help you with this.
Thanks,
Anthony
