SPA phones use SPCP protocol. It is like a crippled version of SCCP/skinny. Presumably, Cisco wants these cheaper phones only to work on UC3XX and UC5XX systems and encourage you to buy more expensive phones for CME and CM..
However as other posters said, you can flash them with SIP firmware and then treat them as third party SIP phones.
However, as third party SIP you should expect some feature deficiencies compared to supported phones.
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