I have a Linux firewall (debian) and recently we setup new phones from Vocalocity. When connecting the new phones they would not authenticate using SIP and after investigating we found out that with the phones using an outbound port of 5062,5063,5064 there would be no reply. As soon as we changed the phones to 5073,5074, or 5075 they would work fine. The port that the phones communicate with Vocalocity is port 5060 and that works fine when the phone uses port 5073, 5074 or 5075.
My first thought was there was something blocking those ports in iptables. But there was nothing in iptables blocking at all. Further increased logging showed traffic going out but nothing was ever received back. As soon as the port was changed to 5073, 5074 or 5075 there traffic was replied to successfully.
Previously I had tried to install asterisk to see how it would work so my thought is there is something related to that which is interfering with communication. I have another office with the exact same setup that has no issues so I am confident this is the case. I have checked to make sure all of the asterisk files have been purged and there is nothing listening on the blocked ports at all. I even did an apt-get install asterisk on the system working correctly so I could purge all the packages it installed along with asterisk itself.