The biggest problems is power. The phone company power the POTS line — you can even make an emergency call from an abandoned building with no electrical service (and no phone subscription) if you find a jack and plug in. Under those huge central office buildings are huge banks of -48 V batteries which are required to provide a week of power (or was it two in the US? I can’t remember).
Now you’ll have a router with a battery that the owner doesn’t know about and won’t service, so that within a year or so it won’t work. In an emergency: no phone service.
Now you’ll have a router with a battery that the owner doesn’t know about and won’t service, so that within a year or so it won’t work. In an emergency: no phone service.