One of the more bizarre phones based on the 700 type body was the Loudspeaking Phone No 1. It was not very successful as careful positioning of the sets was necessary to avoid feedback and instability. When talking to another LST1, stability was almost impossible.
This led to the introduction of the somewhat more reliable GEC manufactured Paxmaster or LST2, the design of which harks back to the office intercom sets of the 1950s.
The PO finally introduced the gartantuan LST4. This solved the instability problem by only allowing simplex transmission. It was famous for its omni-directional microphone which picked up the room acoustics and this phone was consequently dubbed the ‘dustbin’ phone, as that is where it sounded as if the caller was speaking from.