I used to install security systems..
The short version is... If using the land line monitoring, the alarm panel itself (which is usually in the utility room, or closet) has a wire going to your outside phonebox. It is tied into your house phones as (Priority). This means that if you happen to be using the phone, or on dial-up internet the alarm panel will kick you off and communicate with your monitoring service.
Wireless backup is just that.. It is a separate device that is usually installed near the panel. If reception is bad near the panel it can be located in the attic which is higher and will get better reception. But we would try not to install ANY devices in attics because of heat concerns. (Except devices like smoke/heat detectors). The alarm panel will ALWAYS try and use the landline first. If there are 3 failed attempts (It will try them back to back to back) it reverts to the wireless comms and will provide the same info INCLUDING the failed landline.
I neat little trick is to run an Ethernet to your outside phone box instead of just a regular phone line. This provides extra pairs of wires. If your alarm panel has extra (Zones) you can use a zone to monitor your home phones as well. If someone were to physically cut your home phones before entering, your alarm will go off. All zones on the alarm panel are (Supervised) circuits. They have a resistor that the circuit sees at all times. When the circuit is broken (Opening a window or door) your keypad will (Chime or Alarm) depending if its armed or not. So by adding a resistor to a separate twisted pair of wires you ran to your phonebox (That you graciously heeded my friendly advice) you are adding protection of your home phones. Just remember that the twisted pair DOESNT get terminated anywhere inside the phonebox. They only tie in to the resistor as a complete circuit. This circuit is terminated at one of your zones inside the alarm panel.
Its been a few years since I have installed systems, but Im sure they are still mostly the same.