https://www.cisco.com/c/en/us/support/docs/smb/unified-communications/cisco-small-business-spa300-series-ip-phones/smb3246-configure-linksys-key-system-on-spa300-spa500-series-ip-phon.html
Configure Linksys Key System on SPA300/SPA500 Series IP Phone
The objective of this document is to show you how to configure the Linksys Key System Parameter on SPA300/SPA500 IP Phone.
configure linksyskey systemseriesipphone