Your covert operations might be as simple as purchasing a device that will log and display all incoming calls to your phone line, also known as Caller ID.
You will need to subscribe to this service from your network provider (which you should note will show up on your phone bill). In addition, you will either need a phone that can display the incoming number or a Caller ID box.
(The name of the incoming caller is not provided with the service in certain countries, including the U.S.). These are not simple devices to conceal. But depending on where and how it is installed, the other phone-using members of the household will not necessarily know it is there. Or if they do see it, perhaps their reaction to its existence will be revealing in itself.
If you are using a covert recording device to monitor your phone line, you can use a touch tone recorder (roughly $100), which will “listen” to dialed phone tones and display the numbers on the screen. Because this device has a microphone built into it, you don’t need to connect it to the recorder. To make use of this device, however, you will need to record the touch tones that are dialed on the phone using a tape recorder or digital recording device, one that is concealed near enough to the target phone. Computer technology, however, might offer you better comprehensive options…