Automated serial number generation system by call

Question

Hi I want to build a system where someone call to a number and system will automatically generate a serial number on the basis of person's call serial and tell the number to the person send a sms also to the number  and their is also a limit if anyone call after 40 or 50 person system will automaticall said there is no available space please call tommorow. How can I build this type of system?