If you are experiencing unwanted/unplanned connections in your ACU-1000/2000/5000/T/M, the primary cause is DTMF detection being enabled.
Simplest solution is to ensure DTMF detection is disabled for all interface modules.
If DTMF detection is desired or necessary, then DTMF detection should only be enabled on the modules they are required on, and assign suitable security levels and PIN codes per the manual.