OTP with L2TP over IPSec VPN

HI, is it possible fot otp to be also activated for L2TP O IPSEC remote access vpn? 

We've got a customer who is requesting give question above, however since its not shown in the otp availabe sections, i dont think its possible. 

Or does it account for all of the ipsec vpn's 

I read that there's a addon, i think it was called DUO? How do i configure it, how do i download/purchase it?

The Sophos we're talking about is running utm 9.7 and is up-to-date 

Greetings

George