Lync Online handles a wide range of devices while still maintaining high quality audio and video. However, certain devices may cause an echo in the call. To reduce the echo for a problematic device, Lync triggers a voice switching mode, which limits the ability for multiple people to speak at the same time.
Make sure that the device you are using is optimized for Lync Online. For a list of optimized devices, see Phones and Devices Qualified for Microsoft Lync.
In some cases, a third-party driver or software might interfere with Lync Online. Make sure that the device drivers you are using are up-to-date.