I found a similar question in the Cisco Support Community that may help with your question.
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It seems like UC560 support 8900 SIP phones with limited features. But what about 7800 phones?
According to the response from Cisco's Kirk McNeill, the UC560 does not support the 7800 IP Phone.
If UC560 support SIP phones, then why it doesn't support 7800 series since 7800 are SIP phones as well.
Based on the first thread you sent to me, UC 560 supports 8900 becuase 8900 is SIP. This discussion thread was from two years ago.
Is there anyway we can validate the compatibility with both 8900 & 7800 series?
UC500 has been end of sale since Jan 2014.
Cisco does not plan any engineering validation on end of sale products. Therefore, no plan to validate 7800 and 8900 series phones on UC500 series.