indexing telephone numbers?

[Teemu Antti-Poika <anttipoi@pp.htv.fi>]

We're trying to upgrade to openLDAP 2.0, and have converted our schema to
the LDAPv3 format.  When I try to start slapd (-q 65535), I get the
following debug output:

line 54 (index           telephonenumber,mail,qmailuser,xservid,mobile
eq,pres)
/opt/sfw/etc/openldap/slapd.conf: line 54: equality index of attribute
"telephonenumber" disallowed

Why can't I index telephone numbers? The telephonenumber attribute in 
my class is inherited from inetorgperson.

Teemu