there has been a session concerning Enterprise Voice with Office365 during Ignite 2015. Maybe you can find it here:
As other users already mentioned, Microsoft has approved that you DON'T need any onpremise server to use this feature. It will also be possible to use existing numbers, MS is working with partners (AT&T) to port existing numbers. Features like transfer,
call forwarding, team call will work with this solution.
A tecnical preview will start in the US in this sommer. Other Countries will follow 2016.
Be aware that MS strongly recomments to use Azure Expressroute (http://azure.microsoft.com/en-gb/services/expressroute/) when consuming Enterprise Voice from the cloud. Using ExpressRoute you will have direct connection to an Azure Datacenter (e.g.
the Skype4B Server there) and a stable, reliable network connection.
As fare as I learned payment will be done by MS over the Office365 portal
3 minutes ago