Do a capture and if you like, send the pcap to me. I have sent email in PM.
As mentioned by several this is almost certainly a NAT issue.
Assuming your handsets are correctly provisioned as they should be if you use ZC and you have NAT and port forwarding correct have you looked at the system SIP settings
PBX Settings->SIP Settings->NAT enter your public IP in the External Host box. Make sure external UDP and TCL ports are 5060. It’s advisable to put the local network address in too
PBX Settings->RTP Settings make sure the RTP start and end ports are forwarded on the router
If you still have audio issues take a trace and download wireshark (https://www.wireshark.org/download.html) to see exactly whats going on
One way audio is not unheard of but in my experience the source usually lies in the setup of the router/firewall