I recommend reviewing the device scanning requirements in
this knowledge base article and the device scanning instructions in
this article.
Keep in mind that, by default, an asset is only generated for a device like an IP phone if the device has one or more of the following protocols enabled: FTP, HTTP, HTTPS, JetDirect, MOB (Managed Object Browser), SIP, SMTP, SNMP (SNMPv1, SNMPv2 or SNMPv3), SSH, Telnet or WMI. If your phones don't have any of these protocols enabled, you can tick Save Pinged IP for their IP ranges under Scanning\Scanning Targets to force assets to be generated for all pingable IP addresses.