DNVT
Digital Nonsecure Voice Terminals (DNVTs) are digital field telephones using a 4-wire interface with signaling at 16kbps or 32kbps. Each wire pair carries data in one direction, i.e. one pair of binding posts at each end is a transmit output, and the other is a receive input. The devices may be powered over the 4-wire interface (common battery mode), or may be locally powered (local battery mode) when used in pairs without a telephone exchange. Not all DNVTs support local battery mode mode.
Tactical DNVTs include:
DNVTs are intended to be connected to telephone exchanges (and in some cases, to each other) with WF-16/U two-pair field telephone wire.
Signaling and operating states are paritally documented in IEEE Transactions on Communications, Vol. COM-29, No. 11, November 1981, pages 1609 through 1614:
The above article is available for purchase, but is not included here due to copyright restrictions. The article provides considerable insight into the DNVT signaling, but does not provide sufficient details to implement compatible hardware.