In reply to a post by MrSaffron:

Modem resync would mean router would reauthenticate but no reason for it to totally reboot

Would the above result in a "xDSL linestate Up" in the Event Log of the the router??

Many thanks for the other comments, very helpful.

In reply to a post by shalom2005:

Would the above result in a "xDSL linestate Up" in the Event Log of the the router??

On fibre the Openreach white box modem is looking after the xDSL line state, so the router doesn't know about it.

Its possible the same router could be used for ADSL so it prints these messages that can be safely ignored if you're on FTTC using the openreach white box modem as well as the router.