Sorry to be the bearer of bad news but you will need to get either 7900 or SPA500 series phones. SIP phones will also work with the UC520, but you won't get all the features and limited support from Cisco.
These phones date back from the Sipura acquisition and have lived a useful life in LVS deployments and hosted environments, and still can be used that way.
If you want to move to UC500 the story hasnt changed since 2007, which is only Cisco 79xx phones (CP-5xx which are EOS/EOL) and our new SPA5xx phones are supported. While the SPA5xx can run SIP or a tokenized version of SCCP we call SPCP (reference https://supportforums.cisco.com/docs/DOC-9551), we only support them on UC500 running SPCP.
Note to buyer: This means the SPA5xx phones can not be used on ISR based CUCME or higher end Communications Manager.
here is a nice comparison you can work from.
Before you invest in SPA5xx phones, make sure you understand the limitations of them as compared to 79xx.
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