Explain how registration retry works

[tsukraw]

Yealink,
Could you please explain how registration and retries work?
Under "Account/Register" there is the Server Expires and Retry Counts.

What happens if the SIP server is not reachable within the retry count?

So here is the situation.
Customer has remote phones. Each night the customers firewall has a automatic reboot set on it so the internet goes down for a few minutes.

Randomly in the morning they will have phones "T46G" that will not be registered.
If we power cycle, change setting, enable/disable account do anything on the phone that would trigger it to re-register the phone will come online.

So what i am thinking is that the firewall is taking long enough to reboot that the retry timeout is hitting so the phone stops trying to re-register.

Is this possible? If not what else could cause this?
Would it be as simple as increasing the retry count from 3 to say 6 or something?

Phone has been through all the builds on V73 and is now on V80 still seeing the same issue.

Thanks