Do you still have a paid monitoring service contract from ADT? If so, and if,
and only if, you have an OBi 200 or 202 model (not a 100 or 110 model), and you are using Google Voice, you can use the OBi as if it was a regular (POTS) telephone line with your alarm panel.
If you no longer have monitoring service, you can still hook up the OBi to the panel, and yes, it will generate the -48VDC that the panel is complaining about now.
By the way, there is a setting for the panel's configuration that will disable the phone line warning beeps, but you'd need the installer code for your panel to change it... out of scope for this forum.