Alarm system loses dial tone on Home Phone

On telus fibre with home phone using the fibre.

Yesterday the alarm system 'lost' dial tone' and starts it's 'trouble beep'.  Other house phones also had no dial tone. Yesterday morning this was at 0430 so I turned off the alarm sound.  Later, at a more civilized hour I rebooted the fibre box (what is it called?) and all was well.

This evening, while on a zoom call using the PC and the Ipad, the alarm system started beeping again.  The wi-fi/Internet kept working.  Strangely, while we were on the zoom call and the alarm warning was sounding I could hear the Zoom conversation on the home telephone.  In the old days you'd call this 'crossed lines'.  When we completed the Zoom call the alarm system kept beeping and the home DID have dial tone. The alarm system shows error code 4 = communication error. Any ideas where I should start? Thank you. Paul