It doesn't really matter - and I don't know who you'd be calling. I don't know for sure, but I would guess that rehab'd loans are sold in batches to servicers per their contracts with the Department of Education based on the availability of funds and whatever they've agreed on. You'll get whoever is up next to take on some new loans.
You'll see folks complain about X or Y servicer for various reasons, but the reality is that if you're pro-active in keeping tabs on your loan and make your payments on time, they are all basically the same - I guess you might prefer one website over another, but they all do the same things. Ultimately, they are all required to play by the same rules, and if you do you have problems, you have the same recourse with any of them by reaching out to agencies like the CFPB and Ombudsman.
Starting Score: 520