If I understand correctly, you are not asking help with configuration of SPA50x but configuration of CUCM 10.x, isn't it ? So it's rather CUCM question than SPA50x one.
Moreover, I see no SPA50x on CUCM 10.x: Compatible Platforms, Tools, and Products list. Such unsupported phone may be connected as generic third party SIP phone only (as far as I know). So your question can be rephrased as "How to configure ad-hoc conference with third party SIP phone on CUCM 10.x". Your chances to receive valuable answer here are rather low.
You should consider a CUCM related community, like IP Telephony.
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