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.
The reason I'm so confused is that when I was in Thailand I used "calling plus" only over wifi just like I used wifi calling when I was on tmo. I never used it other than with wifi. In the US I used it sometimes when I had a poor signal (2 bars) at my home - b/c I thought it was using wifi instead of the cell towers (so I guess that wasn't right) - anyway I want to be able to use wifi calling and at the same time be able to use other apps (eg look at my calendar, browser).
From the answers I guess I will be able to do this both in the US (yes I'm in a rollout area) and while overseas (I only use it in airplane mode only with wifi to avoid unexpected charges)