Currently using a Sangoma S500 with four extensions programmed. Have to use the web interface for the time being because I was having issues with EPM.
Basically, when there is an internet connection drop, accounts 2,3,4 attempt to re-register within the 2 min specified registration expiry. However, account 1 takes around 10 minutes to recover. Everything is identical in all four accounts with the exception of the extension number and password. All extensions are chan-sip and the phone is located remotely behind NAT in ipv4 mode. PBX is not behind NAT.