What type of voice mail are you using?
An Inskin or an external(analog/digital)?
If analog, possible problems with ring generator.

When did it stop working properly? Only when you put the expansion cabinet in? Is the digital card the only thing in the expansion cabinet? Just hook up the expansion cabinet and don't plug in the DTIB card. Is vm still sporatic? Then plug in the DTIB card and see if the same thing happens.

Have you used etrace to see call progress if timers are activating extensions properly?

Are there more than 28 co ports in system? Not lines, but co ports? If so partition out what's not being used.

It's hard to give specific advice not knowing all variables. If you are close to port limits, resources start failing.

I wouldn't expect to get answers to all this stuff but look at any of these items to see when things go wrong.

Tip and ring, the work of the devil!