Hi Manoj,
Unfortunately the 7912 does not have a "built-in" mic, so the speaker is listen only. Have a look at the following excerpt;
The Cisco Unified IP Phone 7912G is designed to grow with system capabilities. Features will keep pace with new changes via software updates to the phone Flash memory. Examples of currently available features include:
Support of a single line or directory number
Calling name and number display
Call waiting
Call forward
Call transfer
Support of Extension Mobility
Three-way calling (conference)
On-hook dialing, predialing, and off-hook dialing
Redial
Call hold
***Call monitor (speaker only, no microphone)***
"Messages" soft key that allows access to voicemail messages
Four speed dials configurable at the Cisco Unified CallManager
From this 7912 data sheet;
http://www.cisco.com/en/US/products/hw/phones/ps379/products_data_sheet09186a00801739c0.html
Hope this helps!
Rob
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