wasn't there a 13 digit ID number on the notice?
anyway, the IRS phone lines where you can talk to a human are either closed or if open expect an extremely long wait because they were shut down for a period and now there are a vast number of people calling and the IRS offices are not fully staffed.
800-829-1040
First choose your language.
Then 2
Then 1
Then 3
Then 2
You should then be transferred to an agent, but there is likely to be a long wait.