After a power outage it may take the ATA 45-60 minutes to register. Once the ONT has finished rebooting you can power cycle the ATA again to attempt a faster re-register.
That has not been my experience as of several months ago. The ATA does not re-register if the modem reboots and the ATA is either still online or they rebooted at the same time (simply because the ATA finishes rebooting well before the modem gets a signal).
Here's my experience with FX1 (resold ATT fiber). Modem got a FW update and the modem rebooted. ATA was still on. I had forgotten about the update and went to bed and forgot to reboot the ATA. When I woke up 7-8 hours later the ATA was still unregistered. A manual reboot using ***999 got it registered again.
I suggest you speak with your voice team and ensure the ATA's firmware is updated to ping your voice gateway every 5-10 minutes--not 45-60--then re-register if it is not. Sonic espouses the value and importance of a land line (and while many young bucks don't agree due to them having cells, I value mine) please keep disruptions to a minimum.