External check and Group Pricing

I have to do a check for a mobile phone number against an external database. The string returns a 1 or 0. I am thinking a button that activate a popup box (input box) in checkout where the user puts in the number, if the number is ok, the user get a discount for that buy. If not the customer dont get the discount.

by "external" database do you mean just another table in your ZC database? or some external system?

do you maintain the contents of this external database? if so, how?

Yes! It is a external database on another server. I do not have control over this database. I got a string HTTP GET http://vg.eurobate.com/vgpass/ismember.php?gsm=MOBILNUMMER this returns a 0 or 1 based on if the customer is or is not a member of this external database.

I have changed my mind. If i could create a button in checkout that opens a popup form where the user types their number, zen-cart checks if this number is a member. Based on this the customer should get a sertan gift card id in return.