One-way audio is an annoying anomaly where one person can hear the other caller, but that caller can't hear them. It is uncommon in non-VoIP telephony, but seems to occur frequently in systems that have just been deployed.
In traditional TDM voice transmissions, the circuits are reserved for two-way voice transmission, but in VoIP each voice stream is independent. Should one of the streams be lost, deferred, or misdirected, the call the results on has one active stream in one direction.
The most common reason for one-way audio is the result of improper negotiation of the RTP voice stream by a server that is behind a firewall - an unaccommodating firewall.
In a future post we will talk about how to isolate the offending firewall and how to work with the system or firewall administrator to set NAT parameters appropriately so that both RTP streams can be negotiated successfully.