Old thread, but thought I'd provide an update in case this helps someone else.

I am still using the Uniden TRU-8866 phone system. I was having a problem with call waiting caller ID (name & number of incoming call would not show up while I was on the line). I verified that this was not a VOIPo issue and started poking around online for solutions. Following these instructions from Uniden's site worked for that particular issue.

However, a nice side benefit was that this also appears to have solved my MWI problem as well! The MWI on my phones now turns on and off when it's supposed to.

Anyway, hope this info is helpful to others.