Lync internal clients have issues with external meetings

Question

We have a pretty easy Lync setup in our company, 2 FE servers in a Enterprise pool, one Edge server (in a pool. we will add edge servers later) and a Reverse proxy (Databases on a SQL server of course). Everything seems to be working well internally. We
have not deployed Enterprise voice yet but that is coming later. Our issue is when our internal clients try to join a meeting from another company. They can join the meeting and IM but as soon as they try to start Voice, they get kicked out of the call. Internal
meetings work fine with voice and video. They can join the meeting using Lync Web App but that is not a satisfactory solution for this. The company that hosted the meeting said users from other companies can join with the lync client without issue. I am a
remote employee and I can join using audio and video from my home. I have tried tracing the issue with the lync client logs but they make little sense to me. My question is, how does the information flow from internal clients to an external meeting> Is
it through the AV service in the Edge? 
  We have opened the firewalls wide open as a test (closed again of course) and it does not seem to be a firewall issue. Can anyone point me in the right direction?

Answer 1

To both of you, thanks but the articles do not help me. We can host meetings in our Lync 2013 organization without issue and it seems that others from outside can join with either Lync client or LWA. Our remote users connect via Lync client without a VPN
and have no issues with IM, Audio, video or sharing. The issue is connecting to another companies Lync meeting from inside our network. You can connect but as soon as you try to enable audio, you get kicked out. I have seen another post that describes the
issue but there is no solution there either. As I said in the original post, we ran a test where we temporarily opened both the internal and external firewalls wide open for the Edge and Reverse Proxy servers and it had no effect whatsoever so I conclude it
probably is not a firewall issue. Since Edwin verified what I believed in that the communication from the internal clients does go through our companies Edge server when joining a meeting with another company, it leads me to believe that this is
a setup issue with the Edge server but I have followed the setup documents and nothing seems out of place. Our edge server is a virtual server. I  am wondering if this could be related to it or maybe this is just the way things are. Can anyone
tell me that their internal users can join a meeting hosted by another company? We have open federation setup at this time and since the company that hosted the meeting claims to have had users from other companies in meetings without issue, I assume their
SRV records work.