The Area being Modelled is :-
Customers and Phone Bills

A. USER REQUIREMENTS : (Major 'Things of Interest' are in bold.
The User Requirements have been defined as follows :-

The Data Model must capture Customers, Phone Bills, Accounts and Payments.

A Customer sets up an Account with a Telephone Company.
He uses the service and charges are applied as detail lines against
his account (e.g. monthly flat usage fee, monthly equipment rental fee,
charges for individual calls).

Once a month an Invoice is generated and sent to the customer.
The customer pays the invoice (not necessarily in full) and the Payment is
applied against (recorded in) his account.

From these User Requirements, we can derive the following Business Rules :-
B. The Things of Interest, ('THINGS'), include :-
B.1 Accounts
B.2 Customers
B.3 Phone Bills
B.4 Transactions, (eg Payment and Refunds).

C. These THINGS are Related as follows :-
C.1 A CUSTOMER can have zero, one or many ACCOUNTS.

D. Other Characteristics of these THINGS include :-
D.1 To be determined.

E. Sample Data includes :-
E.1 To be determined.

F. Typical Enquiries include :-
F.1 To be determined.