I think it should be connected to the Internet with a public IP address. Using NAT will likely cause you all sorts of problems. Putting the SBC behind a firewall (unless the firewall has an exception that simply allows all IP traffic through to the SBC) is also likely to cause problems.
You don't mention what SBC you have, but an SBC is supposed to provide protection up to Layer 7, i.e. it's an application-specific (SIP) firewall device, beyond the capability of "general-purpose" firewalls e.g. Cisco ASA. They are supposed to be hardened against attacks, so there shouldn't be any major concerns about exposing it on the Internet.