Former New York City Mayor Michael Bloomberg’s presidential campaign indirectly used prison labor to make telephone calls, according to a Tuesday report.

Bloomberg’s campaign used third-party vendor ProCom, a New Jersey call center company, to make the calls, according to The Intercept. ProCom reportedly operates two call centers from Oklahoma state jails, where prisoners were making calls on behalf of the billionaire’s campaign to California residents. A source familiar with the matter revealed to The Intercept that incarcerated individuals at Dr. Eddie Warrior Correctional Center were making the calls.

The Bloomberg campaign claimed it did not know prisons were being used to make the calls and said it had terminated the agreement.

“We didn’t know about this and we never would have allowed it if we had,” Bloomberg spokesperson Julie Wood told The Intercept. “We don’t believe in this practice and we’ve now ended our relationship with the subcontractor in question.”

John Scallan, ProCom’s co-founder, said he pays $7.25 per hour to the Oklahoma Department of Corrections, which the agency then uses to pay prisoners to make the calls inside the call centers. According to the state’s Department of Corrections, the maximum wage prisoners can make is $20 dollars per month, however, a second policy stipulates they can be paid up to $27.09 monthly, The Intercept notes.