Initially, you have to learn how operate the door strike that you have for example, some door strikes operate NO and some door strikes operate NC and then you have to learn how much voltage reguire that operate the door strike and you will need a psu accordingly. Firstly, you can tie a cable between the doorphone’s that you have COM part of the device’s relay part and the gnd part of the psu and then you tie a cable between the NO part of the device relay part and the gnd part of the door strike. Finally, when you tie a cable the +12V part of the psu and '+' part of the door strike, the door will be opened successfully. I added a image regarding the connections that I explained. I hope that it helps you. The example above has been given the door strikes operate with 12V DC transformator.