The UCM shared-line implementation for SIP uses some proprietary SIP extensions that Cisco developed.  Access-to/usage of these extensions requires a special licensing agreement with Cisco IPCBU.  UCM does not attempt to use (or respond to) the extensions when interacting with the built-in 'standard' 3rd party SIP device types.  A SIP-extensions licensed 3rd party device will have a custom device-type installer (a Cisco Options Package, .cop, file) which will create a new device on UCM (which can then use SIP extensions.)