Not DOS, but my company has a Windows XP machine with a dialup connection that I have to keep running for obtuse legal reasons.
If it goes down, the company gets fined. It happened in the gap between my predecessor leaving, and me being hired.
Is it a physical machine and not a VM? I already had issues with getting XP compatible spare parts years ago. Had to buy used parts that were already pretty old (so high risk of failing again).
Does it have to be a XP box or is just keeping a serial dialup connection working the key?