Jay, tell them your employer forbids their calls and one of the numbers they pelt is assigned to a mobile phone. By law on both those counts they have to quit. Document all calls and their lies, you might need the records later if you sue.
All: If you are receiving repeated, misdirected, and/or abusive calls from a third party debt collector, you may find relief in the provisions of the FDCPA, the Federal law which regulates collector behavior and provides you simple tools for your own defense. It grants you control of how you are contacted, and collectors can be sued for their crimes. Learn your rights and first response tactics at FTC-dot-gov. See also your state laws for additional support.