When the RJ31x jack is wired properly, the phone line goes into the RJ31x jack,
then to the Line In connectors on the alarm system. If the alarm system doesn't need to communicate on the phone line then
the phone line is interconnected to the Phones Out connectors with a relay in the alarm panel. From there the
phone line goes back through the RJ31x jack and then on to the house phones.
When the RJ31x jack is wired properly and the alarm system needs to communicate on the phone line, the relay in
the alarm panel opens. This disconnects the house phones from the phone line so that the alarm system
has exclusive acess to the phone line. This prevents modems or burglars for that matter from interferring with the
alarm communications.
When the alarm panel is done communicating on the phone line, it will typicall wait a few seconds to make sure dial tone is restored to the line, and then close it's relay, allowing the in house phones to communicate with the phone line once again. When the alarm panle has the phone lines seized, the house phones should hear nothing, as if the line is dead. Note that some cheap alarm panels you will hear the phone communication very faintly.
When VOIP is introduced into a home, typically the instructions from the vendor tells the home owner to disconnect the
house phones from the Telephone Company Dmarc by cutting or unplugging the line at the Dmarc. Then they
instruct the home owner to backfeed the VOIP phone line into a phone jack. This allows all the phone jacks
in the house to use the VOIP circuit. However since the phone line is being backfed, the alarm panel is unable
to communicate since when it's seizure relay opens, the phone line is disconnected from
the Line In connectors on the alarm panel.
Even if the alarm panel is not wired properly, the backfeeding of the phone line prevents the alarm panel from seizing the phone line away from the house phones. With some alarm panels all a burglar has to do is press a few buttons on a phone while the alarm panel is trying to send a message and the communication can be disrupted.
When wired correctly for VOIP phone service, the alarm panel is connected to the VOIP box as if it were
the Telco Dmarc. This allows the alarm panel to seize the phone line properly and ensures
reliable communications that can't be disrupted just by picking up a house phone. Typically this will require
an installation technician from the VOIP provider or an installer from the alarm company to come
onsite to make these connections however.