These are specific to the SPA 508G, but are compatible with the most of Linksys/Cisco SPA series of telephones (Not adaptors)
Q. What and how many ring tones does the Cisco SPA 508G support?
A. The SPA 508G supports 10 built-in and 2 customizable ring tones. Custom ring tones can be downloaded using the web GUI with the link
where <url> syntax is [[tftp|http|https]://][host[:port]]/<pathname>. TFTP, HTTP, and secure HTTP are supported.
If the phone’s IP address is 192.168.2.1, to download a ring tone “music.mid” residing on the HTTP server 192.168.2.100, and store it as “ringtone2,” the user would issue the following link on a web browser that is connected to the same LAN as the phone: