Spark voice can not access firewall

i had tested ,if there is a firewall between two client ,the call can not build,why ?

what port spark used ,when it calling anthoter client ?

Thanks!

it only computer call computer, not run sip phone

i had iopen media proxy ,but still can not always show :

“Voice chat ended:No Media Received. This may be caused by firewall configuration problems.”

what need i do on my firewall ?