One-way voice / audio may occur of the local network, or PBX, is not set up correctly. The most common causes for one-way audio (that is, where you can hear the caller but they cannot hear you - or vice versa) are:
Router gateway setting 'Disable SIP-ALG': for most (but not all) networks, SIP-ALG should typically be set to be disabled, that is, 'Disable SIP-ALG' should be checked. This setting is most commonly found in a router. Check with the network maintainer to ensure that this setting can and should be amended prior to amending.
NAT Set Up: another common cause for one-way audio is incorrect NAT set up and presentation. For example, the PBX / network will request the SIP trunk service to reply on port 'A' but in actual fact it will be listening to and expecting reply back on port 'B'. Check with the network maintainer to ensure that NAT is enabled, where appropriate, and is correctly set up. On occasion, this may required the use of a STUN sever but again, check with the network maintainer.
PBX Configuration: some PBX's - such as 3CX - allow extensions to have options set to determine how audio will be delivered. If one-way audio is experienced then check these settings. In the example below for 3CX, 'PBX Delivers Audio' should be checked.