Sip Extension Re-registration Timer

I have a freepbx box running freepbx 12.0.43 and asterisk Asterisk 11.15.0. I have two panasonic KX-UT133 ip phones at a remote location connected to the main office (where the freepbx box is ) through pptp vpn. Everything works excellently until there is a minor internet hiccup or the router at the main office reboots for any reason. As soon as internet is restored, the pptp vpn is restored with it as the vpn is set to “always on” but not the phones. The phones re-registers after approximately 30mins which leads me to believe that there is a re-registration timer buried somewhere in asterisks. The only way to get the phones to register immediately is to get someone at the remote location to restart them.I have checked the phone’s web GUI THOROUGHLY but didnt see any mention of re-registration time. Does anyone know where such a timer is in asterisk and or freepbx so that i can lower it to 5 or 10 mins for the 2 phones?