Maybe comfort noise will help you achieve what you are trying to do here?

https://freeswitch.org/confluence/display/FREESWITCH/bridge_generate_comfort_noise

https://freeswitch.org/confluence/display/FREESWITCH/send_silence_when_idle

Thanks,

Branden

From: FreeSWITCH-users <freeswitch-users-bounces@lists.freeswitch.org> On Behalf Of sagar malam
Sent: Monday, October 08, 2018 8:04 AM
To: FreeSWITCH Users Help <freeswitch-users@lists.freeswitch.org>
Subject: [Freeswitch-users] No RTP after unholding call

Caller A and Caller B are on call.Caller A puts call on hold for 2 mins.During these 2 mins FS does not send any RTP to firewall due to which firewall closes RTP port of caller A due to udp inactivity

can we configure FS to send something like empty RTP to caller A ?

I did not find any settings in wiki

Thanks in advance