Advanced Calling allows voice & data at the same time on a Verizon 4G network. So if you are on a call, and have Advanced Calling disabled, you will see the phone's data connection drop back to 1X. I believe this also happens if making a call on a 3G connection, even with AC enabled.
Older phones (for the Galaxies, through and including the S5) did not have this problem, as they had 2 radios, 1 for voice, 1 for data. So they could do voice & data simultaneously, even on 3G. But the current phones, including S6 and G4, only have 1 radio, so they need 4G + Advanced Calling to keep a data connection during a call.
Is the symptom that the other device loses its data access? Or that the WiFi connection itself drops? I don't use mine in this scenario (WiFi hotspot for another device), so I haven't gotten to check this yet.