I found at least a couple of devices that will do the fiber to phone conversion directly.
Now the bad news: They run $1000.00 dollars each end. They are used in enviroments where there is the risk of big (many kv) of potencial betwen ground points. This happens sometimes around substations on the power grid.
The way that they isolate the tephone network from those potencials is sending the phone lines via fiber .
Now again they are $1000.00 each end.
The cheap way will be with the FXO and FXS gateway.
I wil go with Don sugestion , since is the lowest price.