39xx phones are not compatible with the UC520 as of right now. It might be in the works but as of the latest software pack, it is not available.
CME 8.8 is not available for the UC520. The latest software pack information is available in this post:
https://supportforums.cisco.com/docs/DOC-9827
Which shows:
Note: Timelines & Releases are tentative and are subjected to change
Software Pack | Release Timeframe | IOS | CME | CUE | Compatible CCA |
---|
8.6.X | 2012-JUN | 15.1(4)M4 | 8.6 | 8.6.3 | 3.2.1 |
8.2.0 | 2011-JUL | 15.1(2)T4 | 8.1 | 8.0.6 | 3.1 |
8.1.0 | 2010-DEC | 15.1(2)T2 | 8.1 | 8.0.3 | 3.0 |
8.0.5 | 2010-NOV | 15.0(1)XA3a | 8.0 | 8.0.3 | 2.2.6 |
8.0.4 | 2010-AUG | 15.0(1)XA3a | 8.0 | 8.0.2 | 2.2.5 |
8.0.3 | BETA1 | 15.0(1)XA3a | 8.0 | 8.0.2 | 2.2.5 (Beta) |
8.0.2 | 2010-MAR | 15.0(1)XA2 | 8.0 | 7.1.3 | 2.2.2 |
8.0.1 | 2010-FEB | 15.0(1)XA1a | 8.0 | 7.1.3 | 2.2.2 |
8.0.0 | 2009-NOV | 15.0(1)XA | 8.0 | 7.1.3 | 2.2 |
7.0.4 | 2009-OCT | 12.4(20)T4 | 7.0 | 7.0.3 | 2.1.1 |
7.1.3 | 2009-SEP | 12.4(24)SB | 7.1 | 7.0.3 | 2.1 |