I had the issue too.  Solution for me was DNS related.  Since we did not have the FQDN of the server in our DNS it failed and to bypass I put it in the local hostfile (good way to test if you dont have access to the DNS servers).  The domain had to m...

2 things:1 "4) Phone A resumes the hold and voilà , the voice quality is now excellent." did you mean video?2, the only things that comes to mind is that that somehow you are hitting a bandwith restriction and when you go MoH (that streams locally) y...