One-way audio is almost always the computer or the network, not the phone system.

Check in this order:
1. Headset: make sure it is properly connected and selected in the app as the input and output device. If you use a Bluetooth headset, try a wired one.
2. Permissions: the operating system has to allow the app to use the microphone. On Mac you enable it in System Settings, Privacy and Security, Microphone.
3. Network: if you are on Wi-Fi, move closer to the router or try a cable. Audio breaks up before anything else when the network is struggling.
4. Another test call: call a different internal extension. If that one sounds fine, the problem is on the external route and it is worth reporting.

When you report the case, write down the exact time, the number you were talking to and whether the problem started at the beginning or in the middle of the call. With that, support can locate the call in the records and review the recording when there is one.