The 8841 is a SIP phone. The ephone configuration for CUCME references SCCP phones only.
To register an 8841 you will need to start with "voice register global" to invoke CUCME for SIP, then a "voice register dn" for the DN, and finally "voice register pool" for the phone itself. Then you should see the model available.
Take a look at the CUCME Admin guide for full implementation steps for SIP CUCME.
Maren