As you all probably know, Microsoft has enabled the possibility of having Dial-in PSTN Conferencing in Skype for Business Online. In order to do this, you either have to have an E5 license plan in O365, or you can purchase the add-on for PSTN Conferencing in your excisting E1 or E3 plan.
PSTN Conferencing is a very valuable add-on to the O365 Skype for Business as it will enhance the possibilities of joining meetings without having to use a client.
To read more about PSTN Conferencing, visit this site on Microsoft.com:
To se which countries have PSTN Conferencing available, check out the list here.