We are using OCS in live environment several weeks already - ITSP Trunk and DID for each user.
My observation is that OCS "remembers" the last number dialed and defaults to this number next time. However, in this case, there is no difference if you dial
user@domain.com or +14785550001 (which is the matching TEL-URI, if the user is Enterprise Voice enabled) or +1 (478) 555-0001 (the Telephone Number properties in AD) - the call always goes to SIP-URI of this user i.e. user@domain.com
Drago