Historically, to deploy a phone you could type credentials directly into the device or use the phone sign-in web page. Recently, a third method was introduced that allows an administrator to work with a technician to provision the phone in the admin portal and then to remotely sign-in the device. Check out the documentation for more information, including details of the phone firmware required.
The following 2 minute video provides a simple demo of the remote provisioning and sign in experience.
Note: The most likely use case for this type of provisioning is the deployment of shared Android devices such as Common Area Phones.