When the control panel sends an alarm, it takes over the phone line to send the signal to us. This is called "line seizure." Simply put, we, as the alarm company, seize the phone line so that communication between your alarm system and our Central Station takes priority.
If the customer does not have call waiting, we will get a busy signal. If the customer does have call waiting, we will reach their answering machine/service. This most often happens when the panel sends multiple signals (as in this example). If panel only sends one signal, then by the time we call them the panel has released the line.
You may contact us if you'd like to note special instructions on your account for us to contact you at an alternate number in case of emergency.