There’s a jolly useful FAQ on PSTN Calling which you might like to add to your list of interesting things to look at.

First, a reminder of what PSTN Calling is: the add-on telephone service that, when combined with Skype for Business Cloud PBX, can become your phone system. The FAQ covers what licences you’ll need as well as interesting issues such as how calls are charged if you travel to another country. The answer? If a user whose Office 365 licence is assigned in the US travels to London, any calls they make back to the US are considered domestic calls.

Find the FAQ here: http://bit.ly/28TU1bA.