VPN traffic appears to be blocked on Mobile Hotspot and USB Tethering connections

I have a Samsung Galaxy S6 & am unable to use my company's VPN via either Mobile Hotspot or USB Tethering.  The VPN connection works fine over my laptop's wi-fi connection from everywhere that I've ever tried it.  I can establish the VPN connection when using my phone, but it doesn't work.  Once connected, the VPN connection settings on my laptop are identical to those as when I connect via wi-fi.  I can ping the address assigned to my laptop's wireless or USB connection to the phone, but I can't ping the associated gateway on the AT&T network assigned to that particular Mobile Hotspot or USB Tethering connection.  I am using Cisco VPN Client 5.0.07.0440 (this version is required by my company).  It appears that the AT&T 3G/4G network is blocking all VPN traffic (or certain TCP/UDP ports) once connected (based on the ping results).  Note that Mobile Hotspot or USB Tethering works fine for general internet (web browser) access but not VPN access, which leads me to believe that ports are being blocked.  Can you shed any light on this issue & advise me as to the steps necessary to make this work?  Thank you.