I've used the website many times to determine dialing patterns while travelling abroad or while calling foreign countries, and its never failed me.
In theory "+" can be used in place of the international access code for whatever country you are in (tells the phone system that you are going to dial another country). This is very handy, as every country used a different code. Its usually 011 or 001. According to the website I reference above, the international access code in Costa Rica is "00". So they correct dial pattern to call the US from there should have been "+13605551212" if using the plus sign.