I have a mixture of SIP phones that I'm using as 3CX extensions. All of them seem to come from the factory with the Registration Expiration set to 3600 (3600 seconds, or 60 minutes). I want to shorten the period, which I know how to do on each phone. However, I have noticed that when an extension registers and specifies that the expiration period should be 3600 seconds, 3CX responds with a SIP 200 OK message that changes the expiration period to 1800 seconds. If I change the extension expiration period to something short (like 600 seconds), 3CX responds in the SIP 200 OK message with an expiration period of 600 seconds. So, is there some configuration parameter in 3CX (version 9) that specifies a maximum extension registration period of 1800 seconds?