Make sure you tell them to stop calling you at work as it is a place for business, and send them a registered letter with signature to communicate in writing only. That will stop the phone calls.
Usually before they decide to sue, they make a call to your employer to confirm your employment details. I do not see this as a verification of employment. It sounds like they found where you worked and want to harass you at work.
According to Mark Silverthorn, a collector cannot state that they will garnish your wages UNTIL they have the judgement. If they don't, that becomes misleading and false information. Make sure you write the time, date, details, and if your company records all calls, see if you can get a copy.
So how much debt are we talking about here?