The icons at the top left of the display indicate that a call is being made to the internal unit programmed as T1.
While waiting for an answer, you can press T2 and T3. On receiving an answer, the display changes to:
The centre key, now , and T4 are enabled. gives access to the context Audio only settings.
T4 has the function of microphone mute
or enable .
The Audio only settings context allows loudspeaker volume control only.