mobile?

I have inherited a phone where I work.I keep doing something wrong but I dont know what.
When I try to answer an incoming call and am on a call
I press the answer button but get nothing . "sound" appears at bottom right hand side of screen and if I press this the phone goes dead. Does anyone know why or what i am doing wrong.
I havent been able to find the instruction book

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I See
October 2009