If you end up building the solution that way(FXO-FXS bridge,) I don’t think it will be pretty. My advice would be to see if you can swap the phone out for an IP phone – if the customer requirement is “It must be fiber” – It’s easier/cheaper/less kludgy to convert Ethernet to fiber than it is to convert POTS to fiber. But I’m an IP guy – maybe the PSTN folks around here have something different to say…