I try to run red5phone. I know it's a application TCP-based, so I think the quality of voice is better.
But, did anybody know about the communication system? full or half duplex?
It's full duplex.
TCP is worse for VoIP (interactive media in general) because of the extra traffic, delay and jitter it introduces. Voice codecs have enough Forward Error Correction and interpolation capability to tolerate a lost packet here and there. Increased transport delay plus buffers needed for jitter can increase lag to intolerable levels in a conversation.
For better voice quality use a "HD" codec instead.