There are probably other names for this, depending on the company offering the service, but alpha dial is fairly common.

-> "Alpha Dial Numbers
It is usually much easier for a customer to remember a name than a number and Alpha Dial takes full advantage of this. A telephone number that spells your name, a business name, or a “buzz” word for your industry, this provides instant company or product recognition.

Having a word directly associated with your company or business is the surest way to guarantee your business telephone number will be the first to be remembered and therefore dialled. Alpha Dial numbers have been highly successful in the United States, Australia and New Zealand and their popularity in the UK is growing at an ever increasing rate."
http://www.ukfreephone.co.uk/pages/our_numbers.php?content_i...

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Note added at 14 mins (2009-11-11 10:53:35 GMT)
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These types of phone numbers are known as 'alpha numbers':

-> "Most Telephone handsets and Mobile phones have Letters printed under the Numbers on the keypad, this means that some Telephone Numbers or part of the Number can be displayed as a Word.

Research has shown that people are far more likely to remember an Alpha Telephone Number over a Standard Telephone Number, or even a Memorable Telephone Number using only numerical characters."
http://www.numbersourcer.com/alpha-numbers.php