We are having the same issue. Last year the e-file return was rejected. It seemed like they couldn't figure out what the voluntary withholding was and the letter asked to provide documentation such as other 1099s as if there was other income we were trying to hide. It took months to get the refund. I think it's due to this article on the Social Security Administration website: https://secure.ssa.gov/poms.nsf/lnx/
(Just type 0205 then 002 then 016 number to end of URL, won't let me post here.)
I called the IRS and spoke to two people to find out how they wanted it handled this year so no rejections, and the answer was always "Mail it in" so this year I am e-filing but going to mail in a paper copy in with a copy of the SSA-1099 attached at the same time to see if I can get it to not reject that way. No e-filing companies seem to allow this document to be attached and don't understand it's being requested now by the SSA.
