POTS lines no audio intermittent

I have two systems with the following:
freepbx 2.11 (stable 4.211.64-9)
centos 6.4
asterisk 11.6.0

One system has a Digium tdm400 and the other has a Digium tdm2400

This does not happen all the time, and we have been unable to catch a call that is doing this.

The problem reported is:

When making an outbound call, the caller hears nothing but silence. The person’s number being called hears ringing, they answer, but neither party hears any audio. Typically the person being called then hangs up after realizing no one is one the line, which disconnects both ends.

Now one customer is complaining that some inbound calls have been exhibiting the same problem, but if they wait long enough the audio comes on. (this just started a few minutes ago)