Mistakes can happen. For offsets though, they usually occur when you had a prior offset on the books (i.e. a past-due student loan that you made current recently) and it was just never removed. Also, you can have an offset for a state tax debt or other government programs such as unemployment compensation repayment obligations.
What you have experienced is called a "courtesy" disconnect. It means the call volume is so high that they won't be able to get to your call within a reasonable timeframe so they disconnect you as a "courtesy". Not sure if you have been calling the IRS (800.829.1040) or the offset program. But, if you haven't called them yet, you should call the Department of Treasury Offset Program at 800-304-3107 and they should be able to tell you at least the agency that the offset went to and then how to dispute it. My best advice is to call on a Wednesday and in the morning. The beginning and end of the week have the highest call volume (same if you need to call the IRS).
The other possibility here (and maybe more likely given the facts of your case) is that you just may have received the wrong reason why your refund was reduced. The IRS can sometimes recalculate your return if they show additional income on their end that you haven't reported or if they believe that you aren't eligible to claim a certain deduction or credit and then they reduce your refund accordingly. So, if you can't get through to the Offset program, then you could also try calling the IRS to see if maybe there is another reason for the reduction (800.829.1040). Either way, you should eventually get a letter, but it can take up to 30 days after your refund was paid.
If this really drags on and you can't get any answers, then you could look into contacting the Taxpayer Advocate Service. They are an independent organization that helps represent people with IRS issues.
https://taxpayeradvocate.irs.gov/
