If any of the following issues are occurring in your environment:
Dropped Calls
One-way audio
Extensions don't ring or they ring unexpectedly
Intermittent registration failures
Solution:
DISABLE SIP ALG (Application Layer Gateway) for SIP and RTSP on routers.
SET UDP Timeout to: 10 minutes
(SIP ALG is meant for good, but more often than not, it causes problems with proper VoIP communications)