In typical analog phone applications, the exchange side modem and the subscriber side modem are used in pair as shown in the above diagram.
These two modems are linked together using multimode fiber or singlemode fiber as per site requirement.
The exchange side modem is connected via analog twisted pair telephone or RJ11 to PBX system and the subscriber side modem is connected via analog twisted pair telephone or RJ11 to analog telephone. The subscribe side modem most probably contains a ring generator whereby it generates a ring tone to the analog telephone.
In most cases the exchange side modem and the subscriber side modem are both able to add in a data modem transmission of type RS422 or RS485. The data modem is connected to both modem via twisted pair cable or RJ 45 type.
There many types of fiber optics modems in the market nowadays. The designer should design his or her network in accordance to availability of fiber optics modem specifications.
I have been in the field of fiber optics since early 1990s. I gained fiber optics skills and knowledge via my working experience as end-user, main contractor and sub-contractor and finally as an optical fiber enterpreneur.