asterisk users - Aug 2003 - screeching and MOH bleed on PRI

Hi guys,
About a month ago, I went from asterisk infancy to diving in to my first 
production level install.  I have a Verizon ISDN/PRI coming in to my 
asterisk box, pretty standard configuration with AT&T analog phones.  In 
the almost 3-4 weeks of evaluating, asterisk seems to perform superbly 
with the exception of a few minor glitches.  Over the next few days I will 
post these to the list, trying to work through them one by one. The one 
that I start with tonight is by far the most serious and disrupting.

Occasionally when dialing from one extension to the other the dialing 
extension hears a very loud screeching and feedback sort of sound wtih 
what sounds to be the musiconhold blended in with it.  This has also 
happened on one occasion when one of the extensions was performing a 3 way 
call.  Once it happens, you never hear the called number ring or anything.  
The sound is so obnoctious all you can do is hangup.  In a related problem 
sometimes when you dial and internal extension (one that normally works 
fine) it never rings, all you hear is music on hold.  

Has anyone experienced any of these things before or have any ideas what I 
might do to correct them?
AJ