Streem Calls when you are connected to Wifi
When you are connected to a wifi network, Streem does not use any data from your cellular plan.
Data usage on cellular network
When you are not connected to wifi, Streem sends data over your cellular network - just like Facebook, YouTube or Netflix would.
The easiest way to think about it is a comparison to a YouTube or Netflix video. With either (Youtube or Netflix) it is between 360-580 MBs (Megabytes) for an hour of content/watching -- the same as a Streem video all. On a 10-minutes Streem Call, it would be between 60 - 95 MBs.
Another way to look at it, if someone had a 2 GB data plan (the US average is 3.9GB per month as of a few months ago) then they could Streem for about 3.5 hours a month while not on wifi.