Some devices don't like to be managed in a conventional way. Think IP Phones. Typically they won't have SSH or SNMP running, so discovery won't have much of an idea what they are. No credentials, no useful open ports, no information.
Here's where an Enterprise license pays dividends. You will need to know a port that the target IP Phone (or any device, really) listens on and it's protocol (TCP or UDP). Once you know that, in your discovery add this to either Custom TCP Ports or Custom UDP Ports. Once catch - make sure to include the ports that exist by default for the Discovery Scan Options you are using. These are already listed, however will be 'greyed out', see below screen shot. IE, if your IP Phone listens on TCP 5001, make your port list 22,135,62078,5001
Once that has been done add a new Rule. Menu → Manage → Rules → Create Rules. Set the port to your IP Phones listening port and set the type. See below.
From now on, whenever you run this discovery, if a device responds on your designated port it will be set to type = ip phone.