SIP trunk callers hear silence. How to troubleshoot?

This morning, inbound callers to our SIP trunk hear silence. I have become reasonably good at configuration basics but do not understand the details of troubleshooting & tracing calls.

The Asterisk logs include this entry over and over:

[2014-09-05 10:55:33] WARNING[10338] res_musiconhold.c: poll() failed: Interrupted system call
[2014-09-05 10:55:33] WARNING[10338] res_musiconhold.c: poll() failed: Interrupted system call
[2014-09-05 10:55:33] WARNING[10338] res_musiconhold.c: poll() failed: Interrupted system call
[2014-09-05 10:55:33] WARNING[10338] res_musiconhold.c: poll() failed: Interrupted system call

A quick internet search for the error gives this PIAF post but nobody in the PIAF forum responded to him.

I appreciate any ideas on how to move forward. Thank you.