The “value” field must contain the ID of the list the user will be subscribed to. You can see that numeric value shown in the MailUp admin console under Settings > Edit lists or looking at the dropdown menu at the top left of the page. In the image, the list called "New products" has value=3.
Mobile phone number (optional)
|0||Operation completed successfully / Either email address or mobile number has been changed|
|2||Invalid email address or mobile number|
|3||User already subscribed|
|-1011||IP not registered|
m = MailUp::API.new('console_url') m.xml_subscribe(:email => 'firstname.lastname@example.org', :list => 1) # => 0
Xmlchksubscriber.aspx (User subscription check)