After collected your baggage and passed through custom inspection please TURN LEFT to reach public area of arrival terminal. Go straight to the end of hall. After collected your baggage you will pass the slide door to access to the public area. Sometimes it is crowded at this area. Please go to any shops or currency exchange counters in the terminal and ask for changing the coin.
Most public phones can call to mobile phone.
Alternately you may buy a phone card which starting from 50 Baht. This is to save your time in crowded Bangkok Airport. Your trip in Thailand will then start from your selected hotel not from a busy airport. Alternately you may take an airport limousine which rates are varied from 650 to 850 Baht depend on the location of your hotel. Airport taxi rate will depend on the meter.