One way audio is most likely caused by the SIP media ports (10,000 thru 20,000 UDP) not getting to where they need to go due to a Firewall or NAT.

Troubleshooting:

• If the VoIP phone is outside your office make sure the NAT setting for that extension is set to yes in the FreePBX Extensions Page.
• If your Server does not have a Public IP and you are using port forwarding to make it available on the public internet, your server is behind NAT and you should set the externip and localnet options in the sip.conf file using Config Edit. The externip option should be set to the Public IP Address assigned to your internet router, you may find this address by opening http://www.ipmonkey.com in a web browser. The localnet setting should be set to your local network and mask, for example if the local IP address of the server is 192.168.1.200 the localnet setting would be 192.168.1.0/24.
  
  
  
  
• Check and double check any firewall that may be blocking ports 10,000 thru 20,000 UDP from reaching the Server or VoIP phone.