We are also having this scenario when we have calls going thru Call Pilot between our Main switch and a satelite switch and back with TAT on, if TAT is off it seems to work, but we must have TAT on to handle our switch configurations for other reasons. Nortel is in our Call Pilot investigating this and have duplicated the problem in their lab. So hopefully a PEP will be coming soon....

You may want to make some test calls internally, and note the Call Pilot port that you are hitting (should appear on the display of the phone) and see if it is isolated to a  port.  Since it is a sporadic problem, you may have an issue with one port.  

Unfortunately when the system drops the calls, it doesn't display which port it was trying to access.  On the ports, it is bounding around pretty randomly when you tranfer from one message to the next.

You maintain the same channel or port during the entire menu service, so if you are dialing an SDN and getting the first menu service, when you press an option off of that menu to go to the next place, in the time it takes to play an announcement or menu choices prompt, you should be able to see catch the port #.  It doesn't change ports each time an option is pressed.  

This is a PBX Call Pilot or is it on a BCM?  

grumpyone60, Have you received a latest and greatest from Nortel regarding this issue.
Have the exact same experience with removing the TAT to make it work.
Cheers!
Pudge99

Sorry that it has taken me so long to get back to the post.  Here is where we are so far:

I have to do some research about the TAT.  I hate to admit it, but I have no idea what TAT is.  I did some looking and Nortel uses the TAT accronym in two places.  Any help on checking to see if TAT is in use and if so, how do you turn it off.  
Another recommendation was to check on the channels to see if one is faulty.  I went through and disabled ports in able to see if all channels work.  I was able to hit the voice trees by each channel (channel lites up saying busy).  If one is sporatic, I haven't seen which channel is the culprit (sp).  After watching for the better part of an hour, each time you go to a diffent level in the message you do go to a different channel.  

Thanks for all the help everybody.

Have not had any success yet with Nortel fixing the TAT issue, will let you know as soon as I hear anything....