Session Border Controller
The Session Border Controller (SBC) is based on OpenSBC. It is usually installed alongside Asterisk and A2Billing on the same server, although this is not mandatory.
The session border controller can be used in a number of roles, but the most important is to act as an RTP Proxy to assist with NAT traversal which is inherent in the SIP protocol.
Once installed, the administration of the session border controller is almost non existent. SIP accounts are added to A2Billing as usual, and the SBC is integrated seamlessly into the overall system.
If load balancing rather than NAT traversal is your aim, then you may consider a SIP Proxy
The cost of adding in a Session Border Controller to a single server is approximately 2 hours consultancy. This includes training and an overview of how it works, and fits into the overall system.