I managed to setup freepbx (asterisk) and was able to set-up a voice call between Zoipher (installed on my macbook) and linphone installed on my iphone.
The current issue is that the standard installation of freepbx doesn't support "iOS push notifications (CallKit)".
This means that my iphone will only receive incoming calls if the linphone app is running (in the foreground). In other words freepbx must be extended to support "iOS push notifications" in order to assure that you can receive incoming calls while linphone app is not running.
