In other parts of the world it’s known as a telephone test set, lineman’s handset, test phone, etc, but to me it’s a butt / butt set / buttinski.
They’re essentially ruggedized, portable telephones, often with an ability to monitor a line without looping it / going off hook, and used by techs and lineys throughout phone networks everywhere.
I carry Fluke TS52 Pro in my toolbox, it’s got a built in voltmeter, waterproof, backlit display and lots of memory storage options.
It’s a really beautiful bit of kit, (only thing missing is a TDR which the next model up has) but I very rarely use it.
The butt set is in my mind the quintessential piece of test gear, and I’ve got a few from various time periods.
The Telecom Ruggabut was launched in 1994/1995 and was standard issue prior to privatization, and was designed in Australia for the Australian market,
As such it features some uniquely Australian features such as detection for 12kHz Subscriber Pulse Metering (used in Payphones), while the “TLB” Button is tone-loop-break, a 100ms pause in dialling,
Prior to the Ruggabutt there was the Versadial and Versadial Mk2. Lightweight, tough and with a handy RJ12 jack for testing subscriber handpieces, these were made in huge numbers, by PMG and then Telecom.
And as far back as my “collection” goes is the Australian Post Office (APO) Telephone Test Handset No. 4, which lives on the Step exchange in my office, and is a simple rotary dial plus speaker, mic, switch and Neon light to denote ringing.