Hello tiaccadi,
welcome to the Polycom Community.
The community's VoIP FAQ contains this post here:
Jul 17, 2014 Question:Can I deploy Polycom® SoundStation® IP Conference Phones with Cisco® Unified Communications Manager (CUCM)?
Resolution: Please check => here <=
The above document explains in detail what limitations are presented when using the 3rd Party SIP device functionality with a CUCM and you would need to work with Cisco on allowing additional features via raising a feature request with them.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Best Regards
Steffen Baier
Polycom Global Services