Port of Call is the last inhabited island a pirate visited.
To return to your last port of call, simply press M (Map) or click on the Globe Icon and click Return. When a pirate logs on, he or she will spawn onto their current port of call. Port Royal, Tortuga, Padres Del Fuego, Cuba, and Raven's Cove are currently the only ports of call in the game.
All ports of call also have Teleportation quests for each island.