It seems like everyone is marrying DSL and VoIP. DSLi of Miami was probably one of the first ones I know of to take a stab at it. (Albeit a failed stab). Now news is in that Mammoth Networks (a large Qwest DSL aggregator and wholesale customer) has joined with DASH to offer VoIP.
In similar fashion, New Edge used to have a deal with Intelliverse that went sour and Intelliverse left the Hosted PBX game. Now NEN has an Application Service that Packet8 and XCast connect to in order to serve VoIP over the New Edge Network.
It's mainly about Quality of Service. VoIP Providers need to have a handle on the transport in order to provide top quality QOS.
Words of wisdom from another article, "For solution providers, it is also important to note that hosted services do not exist in a vacuum, that a whole ecosystem of products and services is required for a hosted PBX to function, ranging from infrastructure to IP phones to connectivity and, of course, security products. All of those items and services can lead to profitability beyond the margins realized with a premise-based solution."