Just remembered that I had same issue with one provider as Anthony described. I think that provider started to send 183 and you get this event in verto.

2016-08-16 23:55 GMT+02:00 Anthony Minessale <anthony.minessale@gmail.com>:
on the inbound leg from verto you need to set the ringback variable.  Your remote pstn call is not sending any inband ringing and Verto does not support ringing indication like 180. 
https://wiki.freeswitch.org/wiki/Variable_ringback



On Tue, Aug 16, 2016 at 2:49 PM, Ítalo Rossi <italo@freeswitch.org> wrote:
file a JIRA with a test case and tested on latest master please

On Tue, Aug 16, 2016 at 1:01 AM, Bipin Patel <bipin@xbipin.com> wrote:
@Ítalo have u got any chance to verify this issue or shall i file a bug request on jira for this?


Regards,
Bipin



-------- Original Message --------
Subject: Re: [Freeswitch-users] VC no ringtone when remote ringing
From: Bipin Patel <bipin@xbipin.com>
To: FreeSWITCH Users Help <freeswitch-users@lists.freeswitch.org>
Date: 8/9/2016, 12:11:22 PM
thanks for the reply, the logs i gave u r with it set to debug level 10 in verto config, what im guessing is it never sends to client with presence disabled in it but the situation changes when u enable presence as then it tries to send using presence but then VC never understands it. i guess checking the code for mod_verto would reveal y it isnt sending the ringing state to client.

ys i understand its cluecon and u guys might be very busy with it, i can wait, not an issue


Regards,
Bipin



-------- Original Message --------
Subject: Re: [Freeswitch-users] VC no ringtone when remote ringing
From: Ítalo Rossi <italo@freeswitch.org>
To: FreeSWITCH Users Help <freeswitch-users@lists.freeswitch.org>
Date: 8/9/2016, 6:35:55 AM
You can enable verto debug logs in verto.conf.xml with param name debug and see if mod_verto is sending back the ringing state, we're in the middle of the ClueCon so it may take some time to answer you



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