In most countries, the dialing code for an international call is 00.

Given that the country code for the US is 1, you would then end up with 00 1 800 xxx xxxx. Any number can be dialed this way, but you will probably get a message saying "The number you have dialed is not free of charge if dialed from outside the United States. You will be charged at international direct dial rates for this call. If you do not wish to be charged, please hang up now."

In some countries, the 00 prefix is not used, it's best to check with the locals beforehand. If you're using a GSM-compatible mobile phone, you can also enter a + as the call prefix, i.e. +1800xxxxxxx