signal-cli -u USERNAME addDevice --uri "tsdevice:/…"
As outlined in the README under the "Linking other devices" section
The QR codes are encoded tsdevice:// links which are presented by ready-to-mate slave devices.
If you want to link a signal-cli instance to an Android, you have to encode the cli's tsdevice link into an qrcode and scan it.
If you want to link an Android to a signal-cli instance, you have to use my fork. https://github.com/Trolldemorted/Signal-Android/tree/multide...
> To be able to receive messages signal-cli uses a patched libsignal-service-java, because libsignal-service-java does not yet support registering for the websocket support nor provisioning as a slave device.
So maybe you can add a smartphone if you've already registered via the CLI but not the reverse? The wording is unclear to me.