How can I prevent an idle connection from closing

[PjotrekSE]

My IPphone adapter needs to keep a connection on port 506x open at all times, but states close it after about 15-20 minutes idle time even if I use the Firewall/Settings/Advanced/"Firewall Optimization" = "Conservative" setting. If set at "normal" only about 1 minute. This results in that "calling in" no longer works. "Calling out" works all the time, and after a callout, there is another "grace" period of about 20 minutes before calling in fails again.
Is there a setting where I can set the state keep time to a longer time just for this server?
I tried the Firewall/Settings/Advanced/"Sticky connections" and the time setting for sticky connections, but that also makes no difference.