MagicJack also has an iPhone and Droid App...so if your peeps in T&T download the free 'MagicJack' App onto their smart-phone, they'll be able to call anyone in the US and Canada totally free.
Also as Tallman and another poster mentioned, 'Viber' and 'Skype' are great options...BOTH of which are also available on your smart-phones.
Calling cards are okay to use also, but 3 mentioned above work pretty great, especially when both parties have strong Wi-Fi signals.
'MagicJack' free smartphone App...Viber App...Skype App.