@book { CustomerRelationshipManagementindeutschenTelekommunikationsunternehmen,
    title = "Customer Relationship Management in deutschen Telekommunikationsunternehmen",
    author = "Anja {Beringer}",
    year = "2003",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832463632",
    url = "https://m.diplom.de/document/221798"
}