Can be a few things:
- you are speechless.
- Firewall/virus software on the server, but I assume you do not have that as you only got one LAN so Firewall might not be an issue.
- Codecs, make sure that the phones have the correct codecs enabled.
- Try to bind the extension to the media server.
Question:
- Do the phones register?
- Can you hear the called party, but they cannot hear you? (or the other way arround).