I am designing a Customer Relationship Manager with Inventory Control for a Mobile/Landline Sales Company.
The database needs to

• Keep track of customers
• See what phones/landline equipment have been sold to customers
• See what phone plans have been connected per customer
• Track stock of phones (phones are not always in stock so need to be ordered)

Do you think this is the best ERD for this database? (attached)

Some of the problems im encountering are
phones not in stock need to be ordered
not all phones are sold with a service
not all services are sold with a phone