They've been doing hosted VoIP pretty much from the beginning, and have matured to the point where the time - and their offerings - are now right for enterprises. I explored this further in the White Paper, which focuses on why enterprises should consider the benefits of hosted services. This may not be the right choice for all enterprises, but I did enough research to know that services like M5 make a lot of sense for the right situations.
M5 has just posted availability of the paper to their website, and you can download a copy by following the link and filling out the basic registration form. I think there's a great market opportunity here, and I'd love to hear your thoughts.