Thank you, I am in the process of rebuilding our current database and I want to make sure that I do it well. The super-entity makes sense. The actual entities in my system are not customers and suppliers, but:
StudentProspects are the primary entity (some 50,000 records), so I may keep them seperate and combine everything else into a super-entity called "Organization"
Sounds very reasonable! You can either have 2 CallHistory tables, or one with an "arc" relationship to Student or Organisation - i.e. it has a FK to Student and a FK to Organisation, but each record can only have one or the other (check constraint).