Cisco 3911 and 3951 SIP only phones can be added to UC520 running SBCS 1.5 (7.0.3 bundle zip) with several manual steps that will take about 20 minutes.
Marcos and I tried this ourselves this morning and got the SIP 3951 phone (3951 is all I had to try but can work for 3911) to register and operate in a SBCS system.
This document shows you how.
Note: CCA will show this phone as NOT IN VOICE NETWORK, even after its fully operational.
