If you haven't figured it out already the first few circuits ( simplest field phone) lend them selves well to quick assembly after the SHTF. All you need is a few handsets, battery's, switches and wire to throw them together.
They would be most useful say over a few 1000 ft from an O.P. to the C.P. better than radio as there is no possibility of intercept unless the enemy has access to the wire and a phone to connect to it to monitor with. You can of course build several sets in advance of needing them (which would be what I recommend). That could be as simple as screwing the bits to a small board or going to the trouble of getting small boxes made for electronic equipment and assembling a more finished device with a PTT and call switches, a battery holder, electronic buzzer and 5 way binding posts for the line. These could be small and much lighter than conventional sets, as they aren't built to military specs nor do they necessarily need to be. Remember they don't need to be perfect, they just need to be good enough.
In the near future I plan to cover 1) wire (a truly vast subject), 2) high voltage signaling circuits (schematics), 3) voice networks (more schematics), 4) specific Military field phones, (principally the EE-8, TA- 43 and TA -312 and the TP-1). and 5) a build an inexpensive knock off of the EE-8 with more modern components. I also will share an inexpensive and simple way to leverage your wiring to allow it to double as a circuit for data networking between computers.
I encourage people with specific questions to go ahead and ask them so I can address those issues in upcoming posts.
If there is sufficient interest I can assemble kits for people to buy so as to fabricate their own phones, baring that I can supply the URLs for companies where you may buy the parts yourself.