PURPOSE: To enable adequate dealing later on by storing the telephone number of a customer in a calling number storage part in the case that a line in a PBX is busy.

CONSTITUTION: When an incoming call from the customer is received by the PBX 10, a switch 11 connects it to an operator 4 through a vacant line. Customer information is registered to a computer system 6 through a calling number storage part 12 by telephone conversation between the operator and the customer. Besides, if the line is busy, the number of the customer is stored in the calling number storage part 12 through the switch 11. Simultaneously with this, guidance purporting 'we will call you back later on' is sent to the customer from a guidance sending part 5.