Automatically
When you are in Sri Lanka you will be automatically switched to Mobitel on your mobile phone.

OR

Manually
If your handset does not support this capability, you would need to set the network manually.  
The network selection differs according to the handset model. Please refer to the user guide of your mobile phone or consult your dealer for details.
In manual mode, your mobile phone will display all available GSM networks in the area, out of which you could select the network of your preference.