“According to Insight Research Corp., in early 2012, hosted PBX revenue might amount to about $500 million annually, expected to grow to about $1.2 billion by 2015, Insight Research predicts. ... That is far less revenue than many had been predicting would be the case by now. “The hosted PBX/VoIP service providers had only garnered less than four percent of the small business lines (seats) as of year end 2010,” Insight Research says.
It continues, "But the base is so vastly different: Circuit-switched voice represents about $132 billion in annual revenue in 2012, while VoIP represents only about $ 14.6 billion in revenue. At those rates, circuit-switched voice declines about $2 billion a year, while VoIP grows about $1.4 billion."
The answer is simple: VoIP is sold as cheaper than TDM. So revenue will decline. Also, switched voice traffic is migrating to other avenues like chat, IM, SMS, cellular and other types of VoIP (Skype, et al).
FYI: "In 2010, operators made on average only $13.21 per user per year from mobile VoIP services." Yikes!