Is there a way that you can track this yourself? I've had three different mortgages [1] in my lifetime and as far as I know it was all completely opaque who/what was ultimately holding the underlying debt.
[1]: First was a decade ago with RMG (a Canadian monoline lender) and the other two with Scotia, the first of which was a fixed mortgage at purchase, and the second was a refinancing.
When your mortgage is sold, you should get a "goodbye letter" from whoever sold it, and a "hello letter" from whoever bought it. This is so you don't keep sending checks to the old place, and have the new people decide that you aren't paying your mortgage.
I don't mean this flippantly, but it's usually whoever you're writing the check to. My first home had the mortgage sold within a week, and someone else started taking the money out of my account.
Use a mortgage originator that keeps the servicing themselves. One of the reasons I usually use PenFed in the US, after a nightmare of fly by night servicers.