You need to make a small manuel change to the settings for Skype for Business meetings in order to enable participants to reach the meeting via a phone number. This is, however, a one-time settings change.
The change will enable everyone in your organisation to access the meeting directly, whereas external participants will go through the virtual lobby. It also means that, as mentioned, participants can call in via the phone number listed in the bottom of the invitation.
When you are about to create a new Skype meeting from Outlook, you will see the button Meeting Options at the top of the invite. Click it.
Pick Anyone in my organization in the field These people don't have to wait in the lobby. Click Remember Settings to save.
You have now enabled participants to access the meeting via the phone number at the bottom of the invitation.