No, the SPA302 is not Compatible with the UC300 or UC500 (SBCS)
Update 9/19/2015: Cisco published the End-of-Life announcement for the SPA232D and the SPA302 today. We DO NOT recommend selling these products any more as phone support is only available on them until 12/16/2016. Deploy with caution!
Update 1/9/2014: Last summer Cisco published an End-of-Life announcement for the UC320W […]