I would probably do the following:
mapping load ID_Number, Call_ID from TableA;
Acc_Number, Call_ID, ID_Number,
Acc_Number & '|' & Call_ID& '|' & ID_Number as Key
applymap('MapAtoC', ID_Number, '#NV') as ID_Number
where not exists(Key, Acc_Number & '|' & Call_ID& '|' & applymap('MapAtoC', ID_Number, '#NV');
and removing the tables A + C. If you want to keep A you will need a combined key to avoid synthetic key - also you might want to handle possible duplicates in a different way - its just a suggestion how you could approach to this topic.