AFAIK, if the connection to the VPN server is properly set up, the ISP is not able to see any of the traffic.
Nevertheless, I just found a worrying comment to this blog post:
I Just wanted to double check with the experts of this website: is it true what the guy states in his comment?
That is, if I am routing all my traffic through a mobile hotspot with a prepaid sim card, using a VPN on both devices (say a laptop and the mobile phone acting as hotspot), will the ISP be able to see anything but a connection getting into the VPN?
The answer to this question would have been a sound NO, but this guy adding the details of a “carrier server” and a prepaid SIM made me doubtful, as he seems to know more than me.