BLF NOTIFY messages not always working

I was wondering on the Oracle ACME SBC if there is a way to modify the SIP SUBSCRIBE expires timer.  What I am seeing on some customer phones that we have BLF configured is that the phone is over the Internet behind NAT doing TCP SIP.  The phone will come in an register and then subscribe to the BLF.  My server replies back with a 200ok and expires of 3600.  The ACME forwards that to the client still with an expires of 3600. 

Most customer's seem ok with this as I am assuming their firewalls allow a TCP session TTL of 3600, however I do see some customer's where the source port changes.