If you would like to configure a phone number, you will have to first enter your SIP information from your telephony provider in the integration page under the BYO SIP section:
Once your information is filled in, go to the phone number page and create a phone number, and add it as a BYO SIP trunk.
You can check out this guide:
for details on integration.
Note: This is a standard guide used by many telephony providers, so it should be helpful.