AT&T 8510T manual For turning off and then back on again the microphone

Mute button	For turning off (and then back on again) the microphone
	of the handset or the speakerphone so the other person
	on the call cannot hear you.
Redial button	For automatically redialing the last number that you
	dialed from the dial pad.
Softkeys	The 4 buttons located below the display and labeled with
	arrows ( [ ▲ ] ), correspond to words on the display
	screen. Softkeys are used along with display control
	keys to access or create a personal Directory of
	frequently-called numbers, a Call Log of the most recent
	incoming and outgoing calls, and such features as
	selecting a personalized ringing pattern, doing a self-test
	of your voice terminal, and setting the speakerphone
	and the clock.
Speaker button	For turning on and off the speakerphone or the listen-
	only speaker.
Telephone number card	For noting the area code and telephone number of your
	voice terminal.

Transfer button	For transferring a call to another voice terminal.
Volume control button	When on a call, used to increase or decrease the receive
	volume of the handset or speakerphone (if on a call).
	When you are not on a call, the Volume control button is
	used to increase or decrease the volume of the ringer
	and the tones.

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