Microsoft announce that Skype for Business Online will be retired on 31 July 2021. Customers will be migrated to Teams between now and then, and starting 1 September 2019 new Office 365 customers will be onboarded directly to Teams. It’s worth noting that this announcement doesn’t affect either the Skype Consumer service or Skype for Business Server. You can find the announcement here: http://bit.ly/2KreuaK with further details, including customer information on migrating to Teams. There’s also a link to Forrester’s whitepaper: “The Total Economic Impact of Microsoft Teams”, or find it here: http://bit.ly/33oWVPW.
Skype for Business Online features and functionality are moving into Microsoft Teams. From a licensing perspective, additional services such as Audio Conferencing and Phone System will continue to be Add-on licences, and from a technical perspective from 1st October, 2018 new Office 365 customers with less than 500 seats will be onboarded to Microsoft Teams and will have no access to Skype for Business Online.
This is a useful FAQ with lots of information on the whole Skype for Business to Microsoft Teams upgrade topic: http://bit.ly/2pL5Rwz.
Some of the Skype for Business Online services are renamed: PSTN Calling is now Calling Plan, PSTN Conferencing is Audio Conferencing, Cloud PBX is Phone System, and PSTN Consumption is now Communication Credits.
This is a useful page to find out more about these services and their licensing: http://bit.ly/2y71dyv, and page 6 of the October 2017 Product Terms summarises the changes.