Methods, systems, apparatus, and non-transitory computer readable media are described for a scalable computer-telephony integration system. Various aspects may include storing sets of call agent login information for several call agents within the computer-telephony integration system and across several independent computing systems in a contact center login database. Additionally, various aspects may include generating several contact center service categories and sets of contact information for each contact center service category, which may be stored in a contact center directory database. When an incoming call is received from a customer, various aspects may include obtaining customer call information from the customer and generating a customer call key, which may be stored as a reference to the customer call information in a contact center customer call information database. The call key may be used to reduce an amount of data electronically transferred between call agents during telephone call transfers.