Instead of using the Grandstream ATA for residential service, a fax line, or any other analog use, you can use a generic device. In order to fully utilize the service, though, you will need to have access to the web interface for the specific device where you will enter the credentials.
You can locate the credentials for the service using the Atlas portal:
You will see the SIP server name, Outbound Proxy, User ID, and SIP Authentication ID. The Password will be randomized at the start, so you will next want to press the Change Credentials button:
This expanded view will let you generate a new password.
Once you have all of these credentials, you will need to enter them into the appropriate fields in your web portal for the device itself.