Office 365 E5 plan was announced earlier this July 2015, As stated in the Official announcement Microsoft planned to release E5 by the end if this calendar year, as stated in this ZDNET post today is the day "December 1, 2015" for its general availability to global customers. Now you can buy E5 from your Office 365 portal and enjoy the latest innovations from Microsoft and add more value to your Enterprise.
Once you Navigate to your existing Subscription page in Office 365 portal and select to add additional subscription, you can see the new "Office 365 Enterprise E5 without PSTN Conferencing" plan now available, You can choose to pay per month or Annually.
As stated in the below referenced Official blog post PSTN Conferencing is available only in US and will be available later to other regions.
Each License costs $35 per user/month and you can assign few licenses to your tenant to explore the new features and switch plans for appropriate users in the mere future as needed.
Access the Office.com E5 plan and pricing page for more details here: Office 365 E5 Plan and Pricing
Microsoft team started to introduce new features available in E5 one after the other in the recent past and made significant announcements through Office blogs post and yesterday blog post was more significant and concludes these announcements and puts things in action.
Review the Blog post here: Office 365 brings significant new value to business customers worldwide
Additionally read the yesterday's ZDNET post below on this topic from Mary Jo Foley
Stay tuned for more updates...