If you want to be in constant contact using your existing number, you will need an international plan. Most service providers will sell you a short-term international plan. If you don’t mind a temporary, local number, you can purchase a SIM card in St Lucia if your phone is “unlocked”. If you don’t need constant connection, a free alternative is to install “WhatsApp” which allows you to make calls and send texts, pictures, and voice messages all via WiFi. Your friends and family will have to install WhatsApp, too.
There is an extremely helpful discussion on cells phones in St Lucia on the Tripadvisor forum.