The International plan will have to be applied on top of an existing domestic plan which will give you full access to calling and texting both internationally and domestically. Additionally you can also check out the $10 Pay-As-You Go International flexibility plan which offers low rates to a selection of countries around the world.
Simple Mobile is a GSM provider which runs on a T-Mobile MVNO. They dont offer handsets and rely on the customer's unlocked devices which you can use their SIM on.