Roaming was occurred under the concept of “one number connected the world” that allows customer easy to going anywhere. Without considering complicated technical aspects of roaming, All operators try to make a contract to permit their customers using other networks worldwide, for instance, Advance Info Service Public Company Limited (AIS) customers going to Brunei Darussalam their mobile phones will show “BRU-DSTCom/TH GSM”. It means that you would have been extra-charged from service operator, for example not including 7% VAT, you will charged 85 baht/min calling to mobile phone using Thailand network roaming in Brunei Darussalam and 55 bath/min calling back to person stay in Thailand; moreover, there is incoming service charge in roaming that receivers have to pay.Dissimilarity, True Corporation Public Company Limited (True Move) costs 12 baht/min calling in Brunei Darussalam and 60 baht/min calling back to Thailand. It realized that it is very expensive roaming rate comparing to other countries, for example from Table 1, DigiCo.,Ltd customers Malaysia going to Singapore by roaming StarHubCo.,Ltd network pay 5 baht/min for service charge, then when calling back to Malaysia it costs 16-28 baht/min depending on destination but definitely cheaper than Thailand network roaming.