You won't get tax help on the phone unless you used the Live version or purchased Audit Defense.
The IRS method is described in publication 536.
https://www.irs.gov/forms-pubs/about-publication-936
If you have one mortgage, and didn't change lenders in the middle of the year, and didn't have an HELOC with a changing balance, the calculation is pretty easy, using the method in the publication. You can do the calculations and send the IRS a letter of explanation with proof of your work (if the IRS was wrong) or you can do an amended return and just use the correct number from the worksheet instead of letter TT calculate it, if TT was wrong.
To see what TT did, there should be an internal program worksheet that you should probably be able to see if you download the PDF containing "all forms and worksheets." However, if you started the amending process, you might have changed your file so that the current state of the worksheet doesn't match what was filed.
