If you want to achieve this using FreeSWITCH you would likely need to modify the source-code and\or create a new module to control this functionality, it wouldn't be a simple task. You're much better off using a SIP Proxy such as Kamailio which allows you to control the flow of the SIP packets your-self through the configuration, although it can be a bit of a learning curve for those not familiar with Kamailio.

Shaun

From: FreeSWITCH-users <freeswitch-users-bounces@lists.freeswitch.org> on behalf of igor.potjevlesch@gmail.com <igor.potjevlesch@gmail.com>
Sent: 18 December 2018 10:56:03
To: 'FreeSWITCH Users Help'
Subject: [Freeswitch-users] Relay a REGISTER

Hello,

I'd like to relay a REGISTER received on leg A to the leg B. The idea is to have a Contact URI with the B leg IP address when the REGISTER goes out of the FS.

But I didn't found how to avoid Freeswitch to handle the REGISTER locally.

Regards,

Igor.