Companies like vmbc and voiceshot deliver broadcast voice messages
(recorded messages that are played when a person answers, or are left
on an answering service). I assume that these compete with traditional
call centers, but without a live person involved.

I am interested in 
- what applications there are for these services (i.e., what customers
use them, and for what purposes)
- what regulations are relevant 
- any estimates of market size, or research reports on the market
- a list of major players in the market

In terms of market size & regulation I am most interested in the US market.