What is the contact staff id for that?
SELECT T2.contact_staff_id FROM Fault_Log AS T1 JOIN Engineer_Visits AS T2 ON T1.fault_log_entry_id   =   T2.fault_log_entry_id where T2.fault_log_entry_id  =   ( SELECT T1.fault_log_entry_id FROM Fault_Log AS T1 JOIN Engineer_Visits AS T2 ON T1.fault_log_entry_id   =   T2.fault_log_entry_id GROUP BY T1.fault_log_entry_id ORDER BY count ( * )  DESC LIMIT 1 ) 
contact staff id for the fault log entry with the most engineer visits;