Historically, General Telephone/Automatic Electric sold their equipment to (1) independent telephone companies and (2) to businesses seeking a private internal system that was not connected to the outside Bell network.
But in 1972 GTE/AE was advertising their systems to business in direct competition to Bell, as shown by the following ad in a business magazine:
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(two pages)
Also, in that issue Bell advertised its 800A PBX
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