You should be able to create the following auto action to do that. Please keep in mind Joomla requires username, email, and name to create a valid user so you will have to supply all 3. They can however all 3 just be set to email address if that's all you have.

Global
Triggers: None
Type: Registration
User: Automatic
Access: Everybody
Conditions
Field: Custom > Value
Custom Value: [get_token]
Operator: Equal To
Value: TOKEN_VALUE_HERE
Action
Username: [get_email]
Email: [get_email]
First Name: [get_email]

With the above we're securing the auto action using a token so random people can't just guess it via the auto action id. Replace TOKEN_VALUE_HERE with whatever token you like (e.g. 5FirN3RTgGhDXwjHc) as it's basically a password. Use the "Fields" usage to set custom fields values. For example with your mobile phone number you might have the below.

Field: cb_phone
Value: [get_phone]

The get_ in your substitution lets you access parameters from the URL. If your external system is sending this information with a POST use post_ instead of get_. Below is an example of what your auto action URL to be used by your external system. You can find this URL under the Global tab after saving your auto action.


Now we need to add our custom information to this, which would look like the below.