Calling plus is not wifi calling, it was sprint's half hearted attempt at implementing VoLTE with no QOS. Think of calling plus as VOIP over LTE, with no QOS.
Assuming you are in a soft roll out area, sprint will enable VoLTE on your Android and yes, you will be able to use both voice(which is now just data) and data at the same time. Depending on where you are/were over seas, you probably were on LTE which is a GSM technology, so by design it was all data rather than separate voice and data.