The letters simply correspond to the numbers in the numeric keypad of cellphones. One must make sure not to confuse I or O with 1 or 0.
In a case of typosquatting, 1-800-HOLIDAY (Holiday Inn) was mistaken for 1-800=H0LIDAY, where a rival company was able to get commissions which would otherwise go for the more profitable travel agent.

With phonewords, one can overdial, as in the case of 877-WHY-ISLAM (The M is an extra letter).

The use of phonewords began to decline after the supreme court in US ruled against the use of 1-900 numbers for adult services. The phone companies did not want to become associated with adult services. They were politically sensitive, and politicians didn't like this. That, along with the rise of the internet made people less interested in fancy numbers.
(https://www.marketplace.org/2021/03/25/why-dont-we-use-900-numbers-anymore/)

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