If there is a phone number on their letter, use that. Otherwise:

IRS:  800-829-1040  hours 7 AM - 7 PM local time Mon-Fri

Listen to each menu before making the selection.
 
First choose your language. Press 1 for English.

• Then do NOT choose the first choice re: "Refund", or it will send you to an automated phone line.
• Instead, press 2 for "personal income tax".
• Then press 1 for "form, tax history, or payment".  
• Then press 3 "for all other questions."
• Then press 2 "for all other questions."  It should then transfer you to an agent, but there is likely to be a long wait.