While written for the alarm industry this article is generic enough to answer most if not all questions related to this question.

In order for ADSL and a digital dialer to work together properly, the ADSL signal must not pass through the alarm control panel or digital dialer. There are two ways to accomplish this. The first and preferred method is a home run with a splitter. The second is to use a specially designed filter installed in-line between the alarm systems RJ31x jack and cord. Installing a home run is not difficult and, if your alarm technician has the knowledge and skills to properly install an RJ31x jack, your technician can install a home run and splitter.

There are two ways to accomplish the installation of a home run and splitter. The first way is to install an external splitter outside the home adjacent to the NID. The second way is to use an internal, wall mounted splitter adjacent to the ADSL modem. In most cases, the external mounted unit is the easiest way to go. All your technician needs to do is run a single CAT3, or better cable, from the location of the ADSL modem to the external splitter. Then, following the instructions included with the splitter, re-wire the subscribers inside wiring (IW) through the splitter. The internal splitter is slightly more complex to wire. However, if your technician just considers it to be like an RJ31x jack, only with the un-filtered line going from the NID to the splitter and a filtered line from the splitter back out to the NID and then to the subscribers dialer and finally to the subscribers sets, the installation is no more complex than installing an RJ31x jack.

While placing an in-line filter between the RJ31x jack and cord may seem a lot easier, it requires the alarm installer to be sure that when the dialer communicates with the central station, the subscribers modem does not take errors or lose sync. ADSL subscribers expect an always-on connection and are often running applications, such as servers, that require this always-on connection. While a few errors may be tolerated by the ADSL subscriber, loss of sync is usually not tolerated at all. It is imperative that the alarm installer thoroughly test the alarm dialer to ensure that there is no loss of sync whatsoever during alarm communications. It is also advisable to have the subscriber test their download speed prior to the installation and again after the installation of an in-line filter. If there is any loss of sync or download speed, a home run with a splitter will need to be installed. Many ADSL subscribers keep logs of their download speeds and any loss of speed due to the installation of an alarm system will result in a costly revisit by the Alarm Company and / or the subscribers Internet Service Providers technician to clear the trouble.

A few tips:

Always determine if the subscriber has ADSL service prior to starting the alarm system installation.
If at all possible, install a home run with splitter.
Use only CAT3 or better cable for the RJ31x and home run wiring.
Do not run alarm system wiring in close proximity or parallel with any Telco wiring carrying an ADSL signal, as many alarm systems produce signals that can interfere with the ADSL signal.
Do not run any Telco wiring carrying an ADSL signal through an alarm panel or enclosure containing an alarm system dialer.
Before starting work, have the subscriber check their ADSL service to determine that it is working properly and always check with the subscriber after the installation is complete to ensure that the installation has not affected the subscribers ADSL service.

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Ademco