asterisk users - Sep 2015 - Cannot hear the sound from the sips on called

Dear, sir

I have installed the Freepbx 12 on amazon could and it and asterisk run
successfully. I could registered the sips but I wonder why when we make the
call between those sip, each sip cannot hear the sound talking from each
side ? could you please tell me what I need to config more ?

Thank you, I am waiting for your reply .
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