POSSIBLY: stun free address

Public STUN servers

provserver.televolution.net (out of service)
sip1.lakedestiny.cordiaip.com
stun1.voiceeclipse.net
stun01.sipphone.com (out of service)
stun.callwithus.com
stun.counterpath.net
stun.endigovoip.com (out of service)
stun.ekiga.net (alias for stun01.sipphone.com)
stun.ideasip.com (no XOR_MAPPED_ADDRESS support)
stun.internetcalls.com
stun.ipns.com (out of service)
stun.noc.ams-ix.net
stun.phonepower.com (out of service)
stun.phoneserve.com
stun.rnktel.com (out of service)
stun.softjoys.com (no DNS SRV record) (no XOR_MAPPED_ADDRESS support)
stunserver.org (out of service)
stun.sipgate.net
stun.sipgate.net:10000
stun.stunprotocol.org
stun.voip.aebc.com
stun.voipbuster.com (no DNS SRV record) (no XOR_MAPPED_ADDRESS support)
stun.voxalot.com (out of service)
stun.voxgratia.org (no DNS SRV record) (no XOR_MAPPED_ADDRESS support)
stun.xten.com (out of service)
numb.viagenie.ca (http://numb.viagenie.ca) (XOR_MAPPED_ADDRESS only with rfc3489bis magic number in transaction ID)
stun.ipshka.com inside UA-IX zone russsian explanation at http://www.ipshka.com/main/help/hlp_stun.php

List of 263 public STUN servers from EmerCoin project: http://olegh.ftp.sh/public-stun.txt. Tested 2017-08-08.