Most used area codes for Sri lanka are 11 for Colombo, 81 for Kandy, 91 for Galle, 33 for Gampaha, 47 for Hambantota, 34 for Kalutara, 35 for Kegalle. You can look at the Sri Lanka Telcom website for all the codes
If you want, your top-up rule can be setup in a way that your Credit Card will be charged on a specific date every month by a specified amount. This means you wont overspend on calls.