As with most problems, there was a simple solution.

Simply disabling session timers on the FS sip profile did the trick.

On Thu, May 21, 2015 at 3:46 PM, Oleg Stolyar <olegstolyar@gmail.com> wrote:

When I send an INVITE to FS that contains a Session-Expires header, FS always overwrites it in the 200 OK response with this:

Session-Expires: 120;refresher=uac

How can I stop it? I saw that there is the sip-force-expires variable but I am not setting it anywhere.

If I can't stop it from overwriting the value sent by the client, can I at least change 120 to a different value somewhere? Changing minimum-session-expires and session-timeout did not seem to help.