Port order overview
If you currently have a phone service provider or carrier and already have phone numbers for your users, you will need to create what is called a "port order" that transfers those phone number to Skype for Business Online. Once the numbers are ported over, you can assign those phone numbers to your users.
After you port your phone numbers over to Skype for Business Online in Office 365, Microsoft will become your service provider and you can disconnect your service with you old service provider or carrier.
For complete step-by-step instructions, see Transfer phone numbers to Office 365.
If you need to get more telephone numbers than this, please contact support for business products - Admin Help