Yes, you read that right. Ignore the private tax collectors. These folks who the GOP Congress thought could do a better job than IRS are really powerless to collect from you as they do NOT have the power to levy you to collect the back taxes. They also are essentially "sales people" or debt collectors so they will also try to get people in payment plans who really are hardship cases and should be getting Currently Not Collectible or an Offer in Compromise, instead.
If your tax case is in the private debt collector's hands, a better idea might be to find out when your tax debt to expire and see if there are alternatives to a "payment plan" like CNC or OIC above.