False call unknown (6)
I believe the answer is:
phoney
'false' is the definition.
(thesaurus)
'call unknown' is the wordplay.
'call' becomes 'phone' (phone can mean to call by telephone).
'unknown' becomes 'y' (algebra).
'phone'+'y'='PHONEY'
(Other definitions for phoney that I've seen before include "Fraud" , "Not genuine, insincere" , "Fake, misleading" , "Fraudulent" , "Counterfeit" .)