Is anyone familiar with how home intercom systems (25 yrs old) are normally wired? I was assuming they are normally low-voltage but dont know if that's true (its not functioning so can't test). I am looking to install a Ring doorbell system at a friends house, and can't tell if the existing intercom system is wired into a low-voltage transformer somewhere in the system. Very messy installation so hard to trace wiring. Thanks for any input.