How to setup an L2TP VPN connection without Pres-hared key or Certificate

Question

I have a device (NewRock IP phone) that I want to connect through L2TP to our HQ XG firewall. It supports L2TP but the only parameters it takes is IP, username & password (no preshared key or certificate). 
 Is there a way that I can setup L2TP VPN on my XG firewall that does not require a Pres-hared key or a Certificate? We tried Mikrotik and it has this possiblity but we want to stick to our Sophos decive. 
 Thank you,

FloSupport · Answer

Hi Ali Saman 
 Using this KBA as reference ( https://community.sophos.com/kb/en-us/125446 ): 
 Unfortunately, it's not possible without configuring a pre-shared key or certificate due to the security risk introduced. 
 Regards,