Hi Rixy,
Have you looked into the SDP options as well? You show a scenario where when you force a re-invite (hold/unhold etc) the RTP stream comes back.
The following options are to me usually a default concerning rtp. In the past we had the same issues on certain trunks. By using these options I was in some cases able to solve the audio issues.
Avoid Signaling Hold to the Peer - Yes
Force sending SDP in initial Invite message - Yes
Limit to one Offer/Answer per INVITE - Yes
NAT Keepalive - Yes
Prevent the Use of IP Address 0.0.0.0 in SDP Messages - Yes
And with re-invites the following option might have influence as well but thats from experience since providers I have made connections to don't usually like the Mitel to send inactive.
Suppress Use of SDP Inactive Media Streams - Yes
Let me know if this helps, but you might have probably set these already or have tried them
and yes MBG makes everything a lot easier
Cheers