We recently purchased 21 Linksys SPA 962 phones for our office. We are running these directly from our PoE switch (Cisco 300) to our sipx box which is on the same subnet and switch (sipx is not running iptables). Most of our phone calls work correctly but we are finding that periodically (say 1 in 10 phone calls) we have no audio. There is no NAT in the setup so this should all be relatively straightforward.
The most interesting part of the problem is that it appears to be the phone itself. The reason I believe that it is the phone is because of the attached pcap trace. You can see the first call in this trace is bad and the second one is actually completed correctly.
The negotiation over SIP all seems to work correctly and the phone says what RTP port to use but when sipx tries to communicate to that RTP port the phone returns an ICMP port unreachable! From attaching a syslog to the phone as a debug server it appears that the phone believes it has opened up the receiving port but further down it attempts to send rtp back to sipxecs and it cannot send rtp back but this is because I believe the port failed to allocate/open in the first place.
I have tried all four of the available Cisco firmwares that I could find and they all produce the same issue. I also have tried putting the phone on it's own VLAN, tried SRTP, tried symmetric RTP, tried Sticky 183 and nothing seems to fix the behavior.
Did we get bad phones???
We also have a Grandstream ATA gateway with polycoms attached and all of those phone calls work 100% of the time.
Thanks in advance!