Since the request says “extend a phone from the router” I asume that the phone is a VOIP phone ,one that uses ethernet as the main conection.
There are many,many fiber to ethernet converters out there.
They look not bigger than a cigarrete box, use very little power, ussually from a wall plugged transformer, and it will extend the router port to the remote location.
As a bonus, installing a little hub, you also will have data on the same location than the phone.
When you order the adapter, watch out for the type of fiber (single mode or multi mode) and the type of conector on the box and the fiber pannel to be used.
Ninety nine percent of those boxes are just plug ,look at the green lights and play.
Not plug and PRAY like some other pieces.