Please, I need help! 
I have SIP trunk connected to my Asterisk, every couple of minutes during already established session my SSP sends “UPDATE” message to check if the session is still in progress, is there any option or possibility to force Asterisk to send “200 OK” on “UPDATE” message so session can continue?
What is it currently sending? Although unconditionally sending OK would be broken behaviour, changing the 501 to a 200 in handle_request_update will do this. However, getting a 501 response should still tell the other side that the Asterisk is still reachable.
Asterisk should support session timers using re-INVITEs.