Go to

Preferences > Accounts > Add "Other" Account

Then choose “Add LDAP Account”

Server: guru3.eventphone.de
no username
no password

Probably you want a useful description otherwise iOS will use the servername.

Now you should save the settings.

Go to your phonebook and tap on the phonebook name you configured before.

Now you can search for a name

Since the iPhone only searches for prefixes by default you can use a * in the search field.