Your best bet is to listen to your phone vendor and run copper with lightning protection. While I'm not familiar with any device that simply converts fiber to "4 pairs," I would guess that they would not work with your NEC system.

You could purchase a second system and use the fiber via VoIP to tie the two systems together, depending on the NEC system you have. You could also simulate a T-1 via the fiber and do the same thing. However, your best bet would be copper lines. Hopefully, the fiber is in a conduit.

Best of luck.