Voip authentication :(

Hi everyone ;-),
Hope to be happy :>
I have a problem in implementing a scenario, like a simple authentication on voip infrastructure, based on asterisk voip server.

imagine that we have 3 customers, and an operator.
our operator has an extension defined on the asterisk and the number 147 i mean his extension number is 147,
and our customers are like below :
1-Customer one, with the customer number : 21321
2-Customer two, with the customer number : 41622
3-Customer three, with the customer number : 21923

each customer calls the operator from out of organization.
the scenario is like this :

1- each customer calls the system
2- a sound will be played and says : Hi dear customer, please enter your code
3- customer dials his/her customer number with his/her phone.
4-asterisk server checks if the number exists in the list of the customer numbers or not !
4.1 - if yes -> the operator's extension rings and the customer can contact the operator.
4.2 - if not -> customer will be rejected and he/she wont be able to contact the operator.

NOTICE that all of the customer numbers are stored in a file, with the format like below :
12312#213213#51243#123124#21312#124213#3214123# and etc..
please help me thanks a lot.