Contact the firm which installed the intercom on your entrance. As a rule, address and phone number of the company is written right on the front door from the street, under the door lock. If the information is not there, ask the elder on the porch or the neighbor that initiated at the time collecting money to install an intercom system.
Order your universal intercom key. Offer this service now, almost all workshops on manufacturing of keys. However, note that it is your intercom device key universal may not be suitable, it is a matter of luck. Typically, these keys contain a limited amount of codes is calculated for several different intercoms in the area. But does your entrance in these few, you can learn only by experience.
To make intercom key itself is practically impossible, if you are a hobbyist or programmer. If you do decide, you will need a programmer. You can try to make your own Internet based schemes or to buy.