Letters in phone numbers Problem:

You are making a new phone where people can type in specialized phone numbers, for example, 1-800-program, and they would be converted automatically to a usable phone number, like 1-800-7764726 (for the previous example). 

Your program will recieve a string if any length with numbers, letters and dashes, and convert all the letters to their corresponding numbers.

Here is a keypad, for reference:



Rules:


Your program will receive a string
It will process it and return/print another string
Any language is accepted
Since it is code-golf, the shortest code wins
Bash, 30

Edit: Thank you Doorknob for eliminating 3 chars

tr a-z 22233344455566677778889


Example: