If you want the audio to originate outside of Asterisk then you need some mechanism for capturing that audio, then use the appropriate signaling protocol to negotiate with Asterisk and then send that audio to Asterisk. Of course there are several ways to do this with a softphone being one of those ways.
One other option is you could try using one of the console channel drivers, chan_console, chan_alsa. These can capture audio and playback audio from/to your local soundcard (if you have one available). Once configured and loaded you can then do a “console dial” from the Asterisk CLI.