The transition to Microsoft Office 365 E5 means that you'll be able to have true enterprise voice delivered through the cloud. This includes the ability to dial in and out through the PSTN (Public Switched Telephone Network). You'll be able to use your smartphone or desk phone in your enterprise to call right into a conference.
This is a drastic departure from features offered previously in Office 365. Microsoft now has the opportunity to take the carriers on directly. You'll be able to port thousands of numbers to Microsoft, just like you could do with AT&T. Microsoft will own those numbers and deliver full communications without the necessity for any on premises infrastructure. Everything will be delivered out of the cloud.
Existing users can upgrade in-place on top of their existing Office 365 installation. All the features you're already familiar with (such as IM, presence, email, SharePoint, and peer-to-peer calling) are combined with PSTN completely handled on your behalf. Microsoft may or may not be in a good position to do this, but strong competitors like Google and other upstarts are forcing its hand. It's difficult to assess how well-equipped Microsoft is because it doesn't have the experience (or the baggage) of a carrier.
Rollout is happening on a country-by-country basis, but it'll be here way sooner than you think. Microsoft has currently set a target of Q4 2015 with a likely October timeframe to launch in North America. Western Europe will follow sometime within 12 months of the North American launch. There are obviously massive regulatory hurdles that Microsoft has to jump through to become a carrier. It's still learning how to strike the proper balance between becoming a complete carrier and only taking the required steps to deliver functionality.
It really does appear that we're getting close to release. Microsoft had originally given an 18 month estimate for rolling out E5. The running joke at this past Enterprise Connect Roadshow was that it's been 18 months for the last three years now. One of Microsoft's lead program managers said during the event that they are currently in private beta preview. That means that there are already people getting cloud-delivered PSTN connectivity through Microsoft.
The rollout will definitely be done in a piecemeal fashion. Microsoft will start by getting PSTN-based dial-in connectivity. It'll also do broadcast meetings and cloud PBX, which is the ability to route and handle calls in the cloud. Connectivity will be delivered locally through the equipment you already have (such as Cisco, AVAYA, or any PBX that hasn't depreciated yet). Microsoft will then follow through with the ability to provision the ten-digit dial phone numbers that we're all used to.
Microsoft's transition to the cloud is designed to make everything easier and simpler. Less technical staff will be required to support the infrastructure. However, there will be a cost associated with increased network bandwidth.
There's a bit of a fallacy when shifting any workload into the cloud. You're not just going to relinquish control over to Microsoft and expect no longer to have to worry about anything. Ultimately, problems will still occur and you'll still need a similar level of visibility for troubleshooting. You need to understand where the demarcation line is—is it the cloud that's having a problem, or is it something within your responsibility? Even though Microsoft's solution is going to have a positive effect on overhead, it'll require a different approach.