For Greasemonkey:
// ==UserScript==
// @name More Evil
// @namespace http://phonelosers.org/
// @include http://www.tomscott.com/evil/
// ==/UserScript==
var script = document.createElement('script');
script.innerHTML = 'function parseNumber(num) { return num; }';
document.getElementsByTagName('head')[0].appendChild(script);
It bypasses the number parsing altogether, though, so it doesn't give nicely formatted numbers like usual.
Here is a longer version that parses the numbers:
// ==UserScript==
// @name More Evil
// @namespace http://phonelosers.org/
// @include http://www.tomscott.com/evil/
// ==/UserScript==
var script = document.createElement('script');
script.innerHTML = 'function parseNumber(num) { \
var digits = num.toString().replace(/[^0-9]/g, \'\'); \
var parsed = \'\'; \
if (digits.match(/^0[127]\\d{9}$/)) { \
parsed += \'(\' + digits.substr(0,5) + \') \'; \
parsed += digits.substr(6); \
} \
if (digits.match(/^[2-9]\\d{9}$/)) { \
parsed += \'(\' + digits.substr(0,3) + \') \'; \
parsed += digits.substr(3,3) + \'-\'; \
parsed += digits.substr(6); \
} \
if (parsed.length == 0) { parsed = num; } \
return parsed; \
}';
document.getElementsByTagName('head')[0].appendChild(script);