There is no ambiguity here, nor any real need to telephone the IRS, as the instruction is clearly given in the IRS instructions for the 1040 form. https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
"Married Filing Separately
Check the 'Married filing separately' box at the top of Form 1040 or 1040-SR if you are married and file a separate return. Enter your spouse’s name in the entry space below the filing status checkboxes. Be sure to enter your spouse’s SSN or ITIN in the space for spouse’s SSN on Form 1040 or 1040-SR. If your spouse doesn’t have and isn’t required to have an SSN or ITIN, enter 'NRA.'"
