Show "digits only" version of phone number

#!/bin/bash
# Convert phone numbers to numeric (1-800-T-MOBILE -> 1-800-8-662453)

if [ $# -ne 1 ]; then
    echo "usage: $(basename $0) PHONE-NUMBER"
    exit 1
fi

echo $1 | \
    tr '[:lower:]' '[:upper:]' | \
    tr ABCDEFGHIJKLMNOPQRSTUVWXYZ 22233344455566677778889999