International calls are reasonably cheap. Many internet and tour shops offer telephone services which are cheaper than hotel rates.
Mobile phone coverage in Koh Chang is good. You can use a mobile telephone from your home country if you have international roaming options.
Another alternative is to buy a mobile phone with a prepaid SIM card. You can buy a reasonable mobile phone with pre paid SIM card from as little as 1,500 baht.
You can purchase credit for your SIM card at any seven eleven store and most mini marts.
Popular makes of SIM Card in Thailand are 1-2-Call (also known as AIS) & True Move. Both offer similar call rates and good quality coverage on Koh Chang
More in formation on call rates and promotions can be found by clicking the following links