Enter your credentials.
Select the "Applications" tab, then click on "Extensions" from the drop-down menu.
Click on the "+ Add Extension" drop-down menu, then choose the "+ Add New SIP [chan_pjsip] Extension" option.
Add Extension settings:
Switch to the Voicemail tab if you want to add some additional settings (optional):
Click on "Submit" when you are finished.
After that, select "Apply Config" to reload FreePBX and finalize your changes.