If you are using AT&T in the US and you are oversea, you can call toll-free numbers (e.g. 800 numbers) in the US for free, in most cases. Detailed instructions are in this web page. I copy here the main instructions and the numbers for countries that I’m interested in.
Enter the AT&T access number for the country you are in. An AT&T operator or English-language voice prompt will ask you for the telephone number you are calling. Enter the number you wish to reach.
AT&T access numbers
Country AT&T Direct Code Vietnam 1-201-0288 Singapore – SingTel 800-011-1111