There's no multiplexing because it means putting active equipment at the end of the wire, and that means the company has to build a weather protection enclosure, connect power, maintain batteries, and pay for easement(s), maintenance, etc. It's more cost-effective to have the pair go back to the CO., at least for most single-family homes.
Apartment buildings, especially large ones, are more likely to be multiplexed, since the ILEC doesn't pay for the space needed and it's less expensive to run fiber than copper for the same number of lines.
Yes, you are correct. Most companies use the term "Central Office" to refer to both the telephone exchange equipment and the building which houses it.
Almost all fiber; coaxial cables were retired long ago and are now used only for TV transmission, and even then only in locations where the coax is "retired in place" (as far as trunk usage goes) and there's no demand for digital transmission.
There are separate _circuits_ for SS7, which share the same transmission layer as inter-office trunks, but are always routed to two geographically diverse Signal Transfer Point locations via routes that have nothing in common.
Yes, it's a tandem switch. The definition _used to_ be along "two-wire" vs. "four-wire" switches, but since all digital paths are four-wire, the distinction is less clear now. It's very common for "local" exchages to do double-duty as small tandem offices, e.g., for E-911 switching to a PSAP, and the only real difference between "local" and "tandem" switching is the circuit packs used at the edges, since all digital exchanges have to have "four-wire" (i.e., separate paths for transmit and receive) internal switching anyway.
In the Bell System, SS7 is an overlay on the old MF signalling method, so each CO handles trunk reservations the same way for both signalling methods. The exchange keeps an internal list of which trunks are in use, and assigns a vacant trunk to each inter-office call as needed. Intermediate tandems assign trunks in turn, in a daisy chain fashion, until the call is completed or there are no trunks available.
There is no "database" of trunks; i.e. they are _not_ preassigned at a central "brain" before the call setup is attempted. Each office maintains a local list, and makes its own decision about which trunk to use for the next hop, with the assignments taking place in sequence from office to office.
The modem uses either DTMF or dial-pulse, depending on how it has been programmed, and it dials the call in the same way a subscriber would. The CO is unaware that a modem is being used, either for data or fax.
I hope I have.
William
(Filter noise from my address for direct replies)