I have a customer that has an old KX-TA616, with 6 or 8 sets. They said that before, if they were on a call, and another call came in, the set would ring (audibly), but now they don't. I have been through the installation and user manuals, and they seem to be programmed correctly, yet they do not, in fact, ring, if in use. I've done AT&T/Lucent/Avaya for 30 years- this doesn't look that hard- what am I missing? Thanks in advance.