I'll just chip in my two cents worth - I have worked for a major telco in Australia for the past 7 years; specifically in technical support for mobile.

Roaming is dependant on the type of handset you are using while roaming (for the US it has to be at least a triband), and whether or not your carrier has roaming agreements in the country you are visiting. Of course, you must have roaming enabled on your service for this to work and ensure your carrier removes any IDD barrings so you can call out.

There are also other reasons why this would not work such as an ageing SIM card - check the numbers on the back of the SIM; the 9th and 10th digit relate to the year of manufacture and you want to make sure it's no older than 10 years. You'd be surprised how many people own a 10 year old SIM

When you are OS and the phone doesn't appear to attach to a network, perform a manual network selection on the handset to see which networks it is able to detect. When selecting an available network that your provider has an agreement with, it should display 'home network selected'. If you can't connect, then either your carrier doesn't have agreements with that carrier or, there is a problem with the network.

If you are still having issues, contact your providers technical support department.

Hope this helps.