Depending on your needs and quality of your Internet connection, there are two options:
Set up a VoIP server in your office, or on the virtual server in Singapore. We do not recommend virtual servers in Thailand.
You can use software or hardware phones. We are supplying equipment from Grandstream and Yealink.
If you have a Thai-registered company, you can get a 5-channel Phuket VoIP number (076-xxxxxx).
A private person or a company can obtain Bangkok VoIP number (02-xxxxxxx).