Seeing Names of external LDAP contacts when doing internal calls from extension to extension


I’m having a strange issue with LDAP lookup in an UCM6202 with GRP26xx phones connected.
Since I have activated “incoming calls” in LDAP config of the phones, it seems that they see Contact names of their customers when they do internal calls and the calling extension number is included in an longer LDAP entry.

example: 11 calls 12
12 has the name of a customer in the display wich has the number 8576113948 (11 is included)

I had dc=pbx,dc=com as Base DN first, but changed to ou=Global,dc=pbx,dc=com where only external Contacts are located. but nothing has changed.
Only when i switch off “incoming calls” the real extension names can be seen again.
Has anybody an idea what I have done wrong?
I would be thankful for every little hint:-)