Issue: On the receiving end of a call from the Yealink T4x phone, there is a very noticeable difference in tonal quality/level when switching from handset to speakerphone. If the handset is off the switch hook and the speakerphone button is pressed, the phone is following the handset level. However, that is only the receive side (i.e. what the T4x user hears). For the person on the other end of the call there is a difference that is noticeable.
Resolution: When the handset is off hook (even with the speaker button pressed) the phone is using the mic on handset to transmit the voice to the other party and the voice pickup capacity of the handset mic isn’t on the same level as speaker mic, which causes the volume on the other party to be lower.
The ideal use of the speaker mode is to have the speaker button pressed and keep handset on the hook. Don’t put the handset off hook when using the phone on speaker mode.