To add a SIP client to FreePBX, open the menu “Applications” – “Extensions“, choose for example “Generic CHAN SIP Device” and we indicate the main parameters:
User Extension: 6000 (SIP number)
Display Name: Operator (any name to display)
and click “Submit“.
Done, SIP is added, it can already be registered at the specified number and password.
How to add SIP in the configuration file I described in this article – Adding SIP clients to Asterisk