Sounds like depending on the destination phone number you are calling and what upstream carrier is negotiating the call there may or may not be a re-invite happening at the 15 minute mark on some of your calls. This is a series of packets that establishes a re-negotiation of the connection / call behind the scenes. This usually includes INVITE / 200 OK / ACK. There could be a few reasons why the call is dropping; however it is likely related to a failed dialog on the re-invite in which the re-invite ACK is not correctly happening. This could be the result of a SIP ALG, Firewall, broken user-agent-client (some older devices) incorrectly handle this. It also could be the result of a mangled callid, fromtag, totag, or on older devices (from uri, to uri, etc) .. etc. Just some things to think about and look into.