Original house ringer randomly going off

we have a ring doorbell that is hooked up using the original wiring that was already at our house. So we never have to charge it. Recently, after a year of being here, the original doorbell to the house started going off randomly in the middle of the night. But, the doorbell isn’t wired into anything so there’s no way to really ring that doorbell, since ring cameras don’t hook up to preexisting ringer. At least ours never worked on our Pre-existing ringer it only things through our Alexa. Nobody is also at the door when this happens it’s very random in the middle of the night. Has anyone else ever had this problem? What did you do to fix it?