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Even if you had a refund for 2015 that does not qualify you to claim EXEMPT on a W4 unfortunately. I need to explain that part first to you. You can only claim exempt when last year you had a right to a refund of all federal income tax withheld because you had no tax liability, and this year you expect a refund of all federal income tax withheld because you expect to have no tax liability.
A tax liability means you had some tax showing when you complete your tax return but not all your withholding is needed to cover that. You may get some of your withholding back but not ALL.
A lock in letter can be removed but only by the IRS and they normally will do that after 3 years or if you can show then that your circumstances have changed from the year you were placed on the Lock In status. This would mean you show them that you had more income in that past year than expected and you are correct now in paying tax throughout the year.
You can call the IRS (using the specific number on your letter) or the general number(###) ###-####but if you are trying to claim exempt they will not remove that letter for you.
Please let me know if you need clarification. If you do not then a positive rating is appreciated so I get credit for the response. (look for the STARS or SMILEY FACES)