FaceTime works by connecting to data (either 3G/4G or Wi-Fi).
Only the USA Extreme Plan and the USA Max Plan come with 3G/4G data.
The USA Saver, USA Daily and USA Budget Plan do not come with 3G/4G data (and you can't add any). So you'll only be able to use FaceTime when connected to Wi-Fi.
If FaceTime isn't working on your iPhone or iPad, please follow these instructions:
1. Enable the use of cellular data for FaceTime. Go to Settings, Cellular and then scroll down and make sure that "USE CELLULAR DATA FOR" is switched on for FaceTime.
2. When using FaceTime on 3G/4G please turn off wi-fi on your iPhone or iPad.