IBM 1750, 2750 and 3750 Switching Systems

The 2750 was greatly influenced by the European national telephone authorities (PTTs) who insisted on isolating-line transformers to protect their engineers.

An attached IBM Selectric typewriter let customers change system facilities such as the extension numbers of telephones of repositioned staff.

The 2750 was very quiet as it was transistorised with no moving parts other than the cooling fans in the cabinets, compared with the noisy, electro-mechanical Strowger PABX systems then everywhere across Europe.

Costs ranged from $150,000 to $600,000, plus up to $60,000 for the 3750's large room, air conditioning, and false flooring because of the extensive cabling between the three 3750 cabinets.

Suddenly the phone bill, instead of being a single item, could be allocated to departments and individuals, with details of called numbers and duration.

However, systems were mostly ordered because of the first-time use of touch-tone telephones, call re-routing, short-code dialling etc (see below) — all functions taken for granted today.

In the UK, the first 3750 customer was British Caledonian Airways, quickly followed by Bland Payne Insurance Brokers, Derbyshire County Council and Geest Bananas.

Later many household names installed 3750, such as Great Universal Stores, Grosvenor House Hotel, Rowntree Mackintosh, Selfridges, Kwik Save, American Express, Mobil (a network including 1750s on North Sea oil and gas platforms), Unigate (dairies), Royal Bank of Scotland, Reckitt & Colman, Tesco, CWS (Co-operative Wholesale Society), and Greater Manchester Police (GMP) — GMP installed a 3750 at their headquarters Chester House, and later a network of 1750s around the Greater Manchester area.

Early-1970s computers had hardly any typewriter-like terminals and certainly no screens – the IBM Switching Systems introduced the novelty of simple digital-data capture from every touch-tone telephone extension.

In the 1970s a 3750 toured Europe in three French lorries: a demonstration-suite lorry, a system lorry, and a generator lorry. Here potential customers flocked over several days to St Katherine's Pier near the Tower Bridge in London. The people shown were the three French lorry drivers and five IBM UK staff.
Interior of the 1970s lorry-based 3750 demonstration suite that was driven throughout France, Germany, Italy and the UK. Note the use of a slide projector and the then novelty of push-button telephones .
Centralised IBM 3755 Operator Desks could handle calls for multiple 3750s and 1750s dispersed across a country. They were originally developed at IBM La Gaude .