With a lot of Alarm Installation, the installers run a direct telephone line (or a Cat 5/6) directly to the Telephone Box at the exterior of the house. When you disconnect the telephone service, this line might not be connected to the internal wiring inside your house and thus your VOIP box (ATA) is not back-feeding to your Alarm System Tip & Ring, hence the trouble signal.
If I'm not mistaken, there are two ways (maybe more) of accomplishing this Fake Telephone line. One is to simply connect the Tip & Ring to a separate VOIP Adapter using the telephone port of this Adapter. It might not be necessary to have an internet connection supplied to the VOIP Adapter for this 48VDC to be presented to the Alarm Panel (FYI, I notice that my OBi110 ATA supplies 45 Vdc when "On Hook"). The other option is to connect the Telephone Line from the panel to the existing wiring of your internal wiring which is connected to your VOIP Adapter that is providing telephone service to your phones. This approach might cause the Alarm Panel to try to dial out over your VOIP provider for whatever reason (an alarm event, period test, etc.) and this could occur while you are on the phone.
I'm assuming that once the Telco line was terminated, you disconnect the Telco Line completely from your internal house wiring. Otherwise you could damage your VOIP Adapter, since, although the Telco Line was deactivated, it still carries a certain amount of voltage that will damage your VOIP Adapter. BTW, what is the Model Number of the VOIP Adapter and do you have a spare adapter available that could be used for the first option?
Why is it that the Alarm Company refuses to give you the Installer Code? Doesn't the Alarm System belong to you outright or does it technically belong to the Company that installed the System?
GrandWizard wrote:Telephone "on-hook" line voltage is 48Vdc. The panel is just looking to see if that voltage is present for TLM. You can fake a telephone connection by presenting this voltage between TIP and RING on the panel.