IRS is really slow processing this year. Possibly the letter was issued while the e-file was being "reviewed". Does the client expect a refund - check the refund status from here https://www.irs.gov/refunds

No refund? Has their refund payment been processed?

There is a practitioner hotline, which is only slightly easier to get through on, but if the return has not been processed they may not talk with you. https://www.irs.gov/tax-professionals/practitioner-priority-service-r#:~:text=Our%20Practitioner%20P...

I had a return accepted on 4/14 and the refund received on 6/28.