We're testing our 3CX on a directly interneted connected server using the windows firewall and are having intermittent problems. We get some calls working and some calls just dont connect. Completely random. We're done a SIP trace and it looks like large UDP packets are getting blocked. However we've turned off the firewall completely and it still happens. Even with all extensions logged off, so we can just test the inbound to the 3CX and hear the voicemail...same again, sometimes it connects sometimes not. ISP has confirmed that they don't do anything with SIP traffic ie block it etc. anyone else heard of this?