Its my first post I love my N900 but I did not find anything on the internet about N900 being registered to a CUCM as a SIP phone so I did it and here is how I did if somebody is interested.

On the CUCM.
1. Add a End-user (or use an LDAP user if CUCM i connected to LDAP)
Use your standard User settings.
2. Add a phone
Device - Phone - +Add New -
Phone Type = Third-party SIP Device (Basic), Next
3. MAC Address* = Any MAC Not used or the N900 MAC Address (Settings/About product)
4. Use your default phone settings.
5. Digest User = Select the Digest user (see above).
6. Add directory number and use your default CUCM settings.
7. Click Save and Apply config.


On the N900
1. Go to setttings - VoIP and IM accounts - New
2. Select SIP
3. Username = phone number@CUCM_IPADDRESS (I used the publishers IP)
4. Password = does not matter just put something
5. Click Advanced settings
6. Username (digest user see above)
7. Outbound proxy = CUCM_IPADDRESS (I used the publishers IP)
8. All other settings default and click Done

For me I use the MAC adress of the N900 and I associate the phone to the user it is just nicer but you dont have to.
Its also strange that the password does not have to be correct but I did that but as I said you dont have to.