I’m testing a Polycom IP 321 with OnSip.com, and the calls kept dropping.  It was probably a consistent call duration, somewhere around a minute or 90 seconds.  We’re behind a Cisco 871 router.

I was about to start timing the calls and packet sniffing when I found this thread that suggests the command

no ip nat service sip udp port 5060

which apparently turns off some of the special NAT treatment for SIP traffic.  No more dropped calls!