Cisco 8841 tcp and port not working

hi,
i have a cisco 8841 which i want to connect to my freepbx.
when using udp port 5060 for pjsip, registration works fine.
but switching to tcp does not work.
the relevant parameter in the cisco config file is

1

as soon as i set it to 1 i don’t see traffic.
besdie this i am not able to change the port. communcation is always done on port 5060. i checked firmware 12.8.1 and 11.7.1.

any help?