Office 365 Business Premium
If you have Office 365 Business Premium, you can use Skype for Business to make calls to other people in your business who are on your subscription. For example, if your business has 10 people, you can call and IM each other using Skype for Business.
To make and receive calls from people external to your business, you have two options:
Option 1. Use the free Skype app. If you have a very small business (for example, 1-2 people), using the Skype app is the better way to go. It's less expensive to use for domestic and international calls. You can still hold conference calls, make video calls, and share your desktop for presentations. Check out the rates and payment options.
Note that the Skype app isn't part of the Office 365 suite, so it won't be integrated with Outlook 2016 the way Skype for Business is. This means your contacts in Outlook 2016 won't show up in Skype; you'll need to add your contacts to Skype.
We recommend starting with the Skype app to see if it meets your needs. If it doesn't, then consider the next option, to upgrade your plan.
You have to set a toggle to allow your employees to use Skype for Business to search for Skype users. See Let Skype for Business users add Skype contacts.
Option 2. Upgrade your plan, and buy a Phone System and a Domestic or Domestic and International Calling Plan.
Although Skype for Business Online Plan 2 is included with the Office Business Premium subscription, you can’t add cloud voice calling features. So, we recommend that you evaluate an Enterprise E5 (or Enterprise E3) plan to see if it is more cost effective.
Switch to a different Office 365 for business plan. We recommend that you evaluate an Enterprise E5 plan to see if it is more cost effective.
Buy the Phone System add-on.
Buy a Calling Plan for Office 365: you can only buy this after you buy the Phone System add-on.
This second option is ideal for businesses with several people who make and receive external phone calls.
How to see prices and buy
To get Audio Conferencing and Communication Credits:
Sign in to the Office 365 admin center.
Go to Billing > Subscriptions > Add-ons > Buy add-ons.
You'll now see the prices and option to buy Audio Conferencing, and setup Communications Credits.
To get Phone System and a Calling Plan:
Switch to the E1 or E3 plan.
Go to Office 365 admin center > Billing > Subscriptions.
Choose Add-ons. You'll now see the prices for Phone System, and the option to buy it.
After you buy a Phone System license, you'll see the price and option to buy a Calling Plan.