Since the 1920s actually, for the US. https://en.wikipedia.org/wiki/Telephone_number#United_States https://en.wikipedia.org/wiki/Telephone_number#United_States

Since the 1920s actually, for the US. https://en.wikipedia.org/wiki/Telephone_number#United_States https://en.wikipedia.org/wiki/Telephone_number#United_States >In the most areas of North America, telephone numbers in metropolitan communities consisted of a combination of digits and letters, starting in the 1920s until the 1960s. Letters were translated to dialed digits, a mapping that was displayed directly on the telephone dial.