First go to https://portal.office.com and login with a user that has Administrator credentials.
- Once you have logged select the tiles button in the upper left of the screen, there will then be a grey color "Admin" button to select to take you to the "Admin center"
- Within the "Admin Center" select "Admin centers" labeled #1 in the picture above. Next select "Skype for Business" labeled #2 in the picture. The "Skype for Business admin center" will then open in a new window.
- Within the "Skype for Business admin center" first select "organization" labeled #1 in the picture above.
- Then select "external communications" labeled #2 within the picture.
- Under "external access" is a pull down menu, we recommend selecting "On except for blocked domains" to allow communication with all outside parties as showed in #3 above.
- Optional is to select the option labeled in #4 to allow complete federation with users outside your organization.
Once you have completed the above tasks, be sure to scroll all the way to the bottom of the page and click the "save" button. While there is no published time we are aware of, we have found success in waiting 24 hours for these settings to fully propagate in Office 365.