If you are using the T4S series -> it would be prudent to upgrade to 66.82.0.20 on these devices. There 'seems' to be an issue with audio quality on lower firmware versions. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Are we getting the new firmware's for the Yealink's in the pipeline?  This Thread Talks about the "need" of 82.0.20. Not sure if the T4xS series is having issues on lower firmwares.