A local sim card is much much cheaper than global roaming. Get Celcom or Tunetalk. DO NOT GET Hotlink, Maxis,or Digi – THERE IS NO SIGNAL.
We often have spare sim cards from previous volunteers. You simply need to put in your phone and update the credit. If your phone is locked to an operator (eg virgin or orange)- don’t even bring it. We have spare phones or a 2nd hand phone is less than £20 – you can sell it back to many shops for £10 at the end of your trip. Better than your own phone is a cheap Italk or digichatz card, they allow international phone calls form any phone at very cheap rates.
Or you can communicate at least once per week with blogs, email, skype or facebook.
Bring ziploc bags for your phone especially if it is expensive. Sand is an enemy!