No one whistles in Samoa. Ever. Same applies for Tonga. Instead, as you walk down the street you'll frequently hear a variety of sharp hisses or "pssst!" as people attract the attentions of others nearby.
There are two reasons why whistling is considered a no-no here. First, there are those who believe that if you whistle at night, you risk the ire of ghosts who may visit you in your sleep and punish you (the Tongans believe this too). The second reason is more mundane. Whistling at people is clear indication that you consider them no better than a dog. And given the rough treatment that dogs cop here, that's pretty poor.