The Practitioner Priority hotline to the IRS is a professional support line for the sole use of tax practitioners assisting clients with account related issues. 1-866-860-4259. PPS is available to all tax professionals with a valid third party authorization form, i.e. Form 2848 or 8821.
https://www.irs.gov/tax-professionals/practitioner-priority-service
PPS service hours are 7:00 am to 7:00 pm local time