There's nothing you can plug that will cause a red light, short of ripping the optical cable out of the ONT (and you'd know if you'd done that!).

Basically, the things that can cause that sort of fault are exactly the same things that would cause your copper phone service to fail as well (pole strike, digger etc.).

Not saying this sort of problem can never happen, because it can, but's it's really no more likely (and probably less likely) than an equivalent copper fault that would render the phone useless under the same circumstances.

If your current copper connection provides a satisfactory uptime for you, then an ONT delivered voice line should be fine for your needs. If you've got medical or other critical requirements for communications then you should look at another solution for backup comms regardless of whether you have PSTN or VoIP service. Perhaps this could be a cell phone with external antenna or something similar.

EDIT: locating the ONT in a suitable cabinet to provide protection from knocking it is a good idea to prevent the fibre being pulled from it. Just consider a suitable location to mount it, rather than say beside the TV if it could get bumped.