I'm attaching the working and non-working captures. You can see that in the non-working one, the phone receives the SYN/ACK but does not respond with an ACK. I do not suspect routing since, the same phone is communicating with a different TCP port 20001 on the same server. But this time, the traffic is UDP. I seriously still think , its something to do with the TCP stack of Mitel . I know it's been working for ages, but we do see it pretty obvious here, that with a few TCP options realigned and a different MSS value than the working capture, the phone does not respond with an ACK.