signal-cli -a ACCOUNT register

The ACCOUNT is your phone number in international format and must include the country calling code

signal-cli -a ACCOUNT register

signal-cli -a +971585810934 register --captcha signalcaptcha://signal-hcaptcha.5fad97acxxxxxxxxxxxxxx

signal-cli -a +1231234567890  register
Captcha required for verification, use --captcha CAPTCHA
To get the token, go to https://signalcaptchas.org/registration/generate.html
After solving the captcha, right-click on the "Open Signal" link and copy the link.

signal-cli -a +12301234567890 register
Failed to register: [422] Bad response: 422 : {"errors":["number value is not a valid E164 number"]} (NonSuccessfulResponseCodeException)

signal-cli listAccounts
signal-cli send

signal-cli, --help