Here are some important functions of Virtual Session Border Controller:

Robust Security
Distributed architecture for media signaling
Interoperability between signaling and media elements
Availability, High Performance & Scalability
Analysis and Reporting

Some important details of the security feature are:

Integrated Intelligent Firewall:
Authorized Access:
Encryption:
Protection from DoS and Robocalls:

Read more about Virtual SBC: https://www.revesoft.com/blog/sbc/importance-virtual-session-border-controller/