Some contacts saved in your phone may not have the full, international number to reach them. Ensure you always dial the country code prior to your contact’s number.
Some examples:
For the US number (321) 112-2233, dial +13211122233 (or 004 1 321 1122233 in Thailand).
For the UK, add the +44 country code and remove the 0 in the area code, in order for the call to connect. For example, 07511 223344 becomes +44 7511 223344 (004 44 7511 223344 in Thailand).
For the Australian number 02 8181 4646, dial +61281814646 (004 61 281814646 in Thailand).