I had to just do trial and error as I am not finding any information on these settings. What I found is in my Granstream phone I had to have the Register Expiration field set to a value that falls between the values set in FreePBX Registration times. I also made the Reregister before Expiration set to 0 in the phone.
For Example:
Phone Config - Register Expiration = 5 min
Phone Config - Reregister before Expiratoin = 0
FreePBX - Registration Times - minexpiry = 240 secs
FreePBX - Registration Times - maxexpiry = 360 secs
FreePBX - Registration Times - defaultexpiry = 300 secs
- A big problem I ran into when trying to figure out how these changes affected registration timeout was that asterisk has a hard time letting go of the existing timing session. So when you change it and reboot the phone it may start an additional timer. Then it gets very confusing in the logs. I found the best solution was to make the changes to the phone and asterisk and then unplug the phone for the duration of your Register Expiration time. You will finally get a warning in the log that says Warning Retransmission timeout reached. Then its safe to plug it back in. When my phones were finally working correctly Registered SIP 'ext# at 1.2.3.4 messages in the log actually stopped coming in at all and my phones stayed online and registered.
Hope this helps someone else as it was a huge headache for me.