asterisk users - Mar 2008 - Aastra-Asterisk: 6 beeps then voice quality degrades

I have an unusual and recurring problem since I upgraded to Asterisk 1.4.
Sometimes, mid-way through a call, I hear 6 shorts beeps and then the
inbound voice quality degrades massively.  It sounds like the other user is
a robot...etc.

I'm guessing something (aastra 480 or Asterisk 1.4) is warning me about a
problem....but what!  Has anyone experienced this?

MD