KX-T: Ringing without ringing?

[Bob Puff@NLE]

Hi Gang,

I've got a customer with a KX-TD816 who is complaining that people are 
calling them, but the phone never rings.  It happened to me once when I 
called there.  However, every other time I call, it works fine.  They 
have 4 CO lines, and I've tried all lines repeatedly, and the problem 
does not seem to want to happen frequently.  But it happens enough to 
cause them to lose business.  Any ideas?  Two of the lines are coming 
from a VOIP adaptor, and I thought initially they might be the problem, 
but it turns out that it happens on all CO lines.  All stations are 
KX-T74xx sets, with one cordless on a XDP.  There is a voicemail system 
hooked up that should pick up after 5 rings, which also does not happen.

Bob