For a quick read on the subject of how session border controllers (SBCs) fit into the SIP Trunking ecosystem, we’d recommend you jump over to a recent post at NoJitter.com.
In just a few quick paragraphs, it summarizes what enterprise SBCs do to integrate SIP Trunking from Sprint (or other service providers) to SIP-based enterprise Unified Communications systems.
For example, certain SBC features, specifically those connected with the Back-to-Back User Agent capabilities, can resolve interoperability issues between systems both in initial implementations and in later system upgrades.
The SBC can also play a security role, such as protecting the system in the face of registration floods, in which a large number of phony registration attempts are sent in a short period to a SIP registrar. Without an SBC to watch for patterns in these registrations and stonewall them before they can get to the UC application, the flood of these attempts could crash, or at least slow, the system.