I install, configure, and administer open source billing software's and private branch exchanges built upon the Asterisk platform. I have been installing and administering these billing software's for years. I have a large number of clients, many of whom migrated from a hosted billing software's, or were looking to move away from an older, expensive, proprietary billing software's. Some clients use their a2billing install to handle large volumes of calls, and others use a2billing for its powerful realtime capabilities; most use a2billing for wholesale voip resale and for calling card. No matter the case, a2billing offers my clients compelling advantages over (much) more expensive proprietary billing software's.