hi,
I have done connectors for vtigercrm before,
the connector will work with vtigercrm on-premises installation,
we will need to change code of vtiger as well as freepbx configs.
once done you will be able to do click to call from almost any module,
also be able to see call detail records in vtiger pbxmodule.
kindly let me know if we can discuss it in details.
regards.