Phone: (303) 205-8291 Fax: (303) 866-2037 Hours of Operation: Monday-Thursday: 8:00 a.m. to 4:30 p.m. Friday: 9:00 a.m. to 4:30 p.m. Mailing Address: Colorado Department of Revenue Attn: Collections P.O. Box 17087 Denver, CO 80217-0087