All the iPhone models will work in Sri Lanka in the 3G or 4G LTE networks, when you bring them here and switch it on.
First of all there are two options and you have to select one of them for to operate your iPhone in Sri Lanka.
First one is to select suitable local carrier that has agreement with your carrier. If you can find one, then your phone will work in the roaming mode to dial and receive calls and to send and receive text SMS along with MMS. But you may get a surprising mobile phone when you visit back your home for using your roaming facility.
Second one is to buy a tourist SIM package and replace your own SIM card with the new one in your iPhone, if it can accommodate it.
Using the new Sri Lanka SIM card is very cheap, if your iPhone comes with a SIM slot. Normally the tourist SIM is valid for 30 days only. If you want it to be active for more than 30 days, then you must contact your local mobile phone carriers' office to make your request. Sri Lanka is one of the country which used to provide cheap mobile calls and cheap data plan for the users here.
If your iPhone is locked with a carrier in your home country, don't forget to get their permission to use it in Sri Lanka with local carrier.
New SIM card is available at the Colombo airport and all over the island through the mobile phone companies and through their dealers.
WE suggest you to buy a cheap Smartphone along with a SIM card over here and to use it, if your iPhone don't have a SIM slot. In this way you can save lot of money from the roaming calls and data plans.
Here in Sri Lanka you can buy brand new iPhone from the Apple dealer with the necessary guarantee and these devices can use local SIM card and SIM card that is available in your country.
The newly announced iPhone 7, 6S and the iPhone 6S Plus too can work in Sri Lanka flawlessly. Apple Inc won't allow you to pre-order any of their iPhone through their online website to buy from Sri Lanka. You have to buy one of these in Sri Lanka till the dealers offer them for sale.
Check the iPhone features that will be available in Srilanka. iOS Feature Availability. #siri
Public holidays, Sundays and Poya days are the best time to make low priced calls to any part of the world from Sri Lanka through out the day and night.
International country code of Sri Lanka is +94 or 0094.