You need to use ldapfilter instead.
Go into the SA-ldapsearch app and look at the documentation - it's similar to ldapsearch, but does searches against the event pipeline. You will need to add in src_nt_domain, so something like this should work:
eventtype=msad-account-lockout | eval actor=if(EventCode==4767 OR EventCode=671,src_user,src_host) | ldapfilter domain=$src_nt_domain$ search=(&(objectClass=user)(sAMAccountName=$user$))" attrs="mobile,telephoneNumber" | table _time,signature,user,actor,mobile,telephoneNumber | rename signature as "Action", user as "Locked User", actor as "Changed By", mobile as "Cell", telephoneNumber as "Phone"
Enjoy!
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