I would think that either cucm is transforming the calling number or dial rules are getting applied.
To check this simply take a sniffer trace while in soft phone mode:
- check the sip invite to see what the calling number is that is offered by cucm
- check for ldap requests for lookups in the directory
Make sure you are using cupc 8.0(3), with 8.0(2) there was an issue that the client was mixing up application dial rules and directory lookup rules.