You are very familiar with the SMS (Short Message Service). From Children to grandfather - grandmother, and from traders to the President, most already know and use for sending SMS messages.
Then, how the working principles of SMS, why can send inter-operator?. Of your HP, once in the press send, message will be sent to the station (BTS), then transmitted by Sentral (MSC) to a machine called a SMSC (Short Message Service Center).
If you are still sending in a the same operator, such as the Flexi to Flexi, the SMSC will forward to the MSC that serves the customer will be sent.
When sending to another operator, for example, to Flexi to Telkomsel, the Flexi's SMSC, message forwarded to GSMSC (Gateway SMSC) as the interface to the SMSC Telkomsel. And then Telkomsel forward to next SMSC to the recipient.