You can sue any 3rd party debt collector who calls you. It is an FDCPA violation.
Let them call numerous times. Gather the info. File a lawsuit under the FDCPA.
Each violation can result in $1k to $1.5K and every state has a mirror law of the Federal FDCPA
So, that's double for each violation.
They will stop calling when hit with an FDCPA violation fine or go bankrupt.