It has been a while I haven’t write something ! Anyway, I had an issue some weeks ago on a fresh Skype for Business environment. We were having a “Network error” when trying to share a desktop in a peer to peer context with a federated user (Lync 2013 and Skype for Business 2015).
It was working between two internal users, between an internal and a remote user and between two remote user.
After looking at the firewall logs when performing test, we found that the federated user was trying to access the Audio/Video on port 5*****… You know the range 50000-59999 that should only be used for RTP traffic with OCS 2007 federated users ! So those ports are also used with Lync 2013 and Skype for Business federated users for RTP traffic.