RHC: Sir, be careful with any sort of "splitter" arrangement, It is possible you may mess up the line exclusion feature, or perhaps even disable the alarm's ability to call out to the monitoring station. I suggest you test the alarm to the station just to make sure your wiring arrangement hasn't messed up the alarm...!!
Using a DSL filter, even when not absolutely necessary for alarm transmission assurances, has the advantage of allowing your alarm panel to cut off your phones WITHOUT simultaneously cutting off your DSL service. The DSL service will be totally unaffected by any alarm transmissions and you won't have to wait while it re-synchronizes with the server because it simply won't get cut off. I'm told it works by dividing the frequency bands on the line, allocating the voice transmissions to the lower band while leaving the DSL signal on the higher band unaffected (that explanation may or may not be scientifically accurate; however, good enough to describe the functionality......)
That may or may not be worth the $20 filter cost for you.
I'm well aware of how the splitter works. It will split the incoming line into 2 separate circuits.. One for DSL and one for the phones and the alarm. Running the regular talk line to the alarm panel and from there to the phones will keep that working like it's supposed to. A separate DSL line will run to the modem. No need for those filters on every phone. With this setup, the DSL is supposed to work better.
I've found that if you have DSL and an alarm system, that if you don't use a DSL filter for the alarm panel .... you may find that the alarm system will work when you test it, but you may also find out the hard way, that it doesn't work when you need it.
For the small cost of a filter, it's better to be sure.