When Mercury (the first competitor to BT) gained indirect access interconnect with BT in 1985, it was able to launch services to the residential market. To simplify connecting to the Mercury network, special phones were marketed by Mercury themselves to their customers. By pressing the Blue Button, the phone would dial 131 (usually in loop disconnect in those days), then wait for Mercury dial tone, after which it would pulse out in DTMF the customer account number followed by the digits actually dialled by the customer. Such phones fell out of favour after 1992 when single stage indirect access interconnection was introduced. Then it was only necessary for the customer to prefix the number by 132.
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