SIP Traffic Issues

[rick_muller]

A few months ago I upgraded the firmware version on our Fortigates from 5.6.6 to 5.6.9.  Around that same time we started experiencing strange, intermittent issues with some of our Mitel phones.  Some of the actual SIP devices (conference room phones in this case) would not register properly with our Mitel border gateway server.  This happened very intermittently.  When the issues occured, we would restart the device (which in turn re-initiated the registration process) and eventually the phone would register properly without any changes to config.  We have also experienced intermittent issues with transferring incoming calls.

We are using Fortigate 200E's as our main devices as well as Fortigate 80E's at our remote locations.  Our VOIP phone system has been operating in our network for just over 3 years without issue.  Our phone vendor is of course blaming this on our firewall firmware upgrades.

Has anyone experienced anything like this?  Are there any known changes between 5.6.6 and 5.6.9 that might have an effect on SIP traffic?