No sound when two extension person getting call

Dear all,

I am a new asterisk user. I have just installed asterisk 1.4.18 successful. I made two ext. account and try to get the call. Ext.1 call to ext. 2 and the ring wake up. The ext. 2 pick up the phone and try to talk. Here I get the problem no sound and nothing can hear from each ext.

What the problem here?

Thanks for your helps.