What I do in a situation like that is I choose a phone or two and program DSS buttons for every extension and VM ports as well. So now you can see what extension is going off hook just by looking. I don't know if this is going to help you but it's a place to start. I know you replaced expansion mods etc. Weird problem but for giggles do the DSS buttons just to see what you get. I agree that the next step is to replace the ACS but replace it with an R8 and program from scratch (you won't be able to restore from an R7.) . The R7 had lots of problems.

-Hal

CALIFORNIA PROPOSITION 65 WARNING: Some comments made by me are known to the State of California to cause irreversible brain damage and serious mental disorders leading to confinement.