Epygi SIP PBX system enables a SIP Session Timer on a subscriber by default. This session timer acts somewhat as a keep-alive for the phone call. Our switch will either not reply to this or not respond quick enough. To fix this, disable the session timers on all subscribers that are peered to Thinktel. Our switch maintains SIP keepalives to the Epygi PBX so this is not needed at the PBX.