Its just part of the settings when setting up a new call,,, this was covered very recently on this mailing list. Try checking the archives for the thread

From: freeswitch-users-bounces@lists.freeswitch.org [mailto:freeswitch-users-bounces@lists.freeswitch.org] On Behalf Of Arsen Manukyan
Sent: Monday, April 18, 2016 3:17 PM
To: FreeSWITCH Users Help <freeswitch-users@lists.freeswitch.org>
Subject: Re: [Freeswitch-users] help, with Verto - verto-min.js configuration

any other help ?
How to fix setting for turn server ?