This is the scheme of spreading load across several servers of a cluster.

Server1 performs role of signaling load balancing engine for Carrier1.
Its functions:

• authorize inbound call;
• redirect signaling streams of calls to Server2 and Server3 using SIP "302 Moved Temporarily" message;
• GUI configuration changes;
• graphs generation;
• periodic actions (email generation, invoice generation, etc).

This method is for SIP technology only.

Server2 and Server3 act as SBCs (Session Border Controllers).
Their functions:

• routing and hunting;
• balance control;
• call limit control;
• signaling protocol translation;
• when a call is established, maintain the signaling session with Carrier1, Carrier2 or Carrier3;
• media stream proxying;
• media stream transcoding;
• media features (if enabled);
• billing;

After billing has been performed on Server1 and Server2, they store billing data on Server1.

The maximum capacity of this scheme is determined of a maximum capacity of load balancing engine of Server1.