From what I know, MTS does not throttle speeds, but I am pretty sure I know what you are talking about, since this happened to me a number of years ago. Apparently their system (for lack of a better term) is "self-healing". If your getting too many errors it automatically adjusts to be more stable. That is likely why you never noticed a problem on your VOIP, as the system just fixed itself when it detected problems.
You would not have got an email or call, because it is not something that is monitored.
I asked them to do the same thing for me which was put my service back up to a faster profile manually, while they will do that, it was explained that at the faster speed my line would be more prone to errors and the end result is usually re-transmission of dropped packets and in effect slower speeds even with the faster profile. Sure enough, I ended up with more problems at the faster profile that I did with the slower profile that I was automatically switched to, and in the end I had them move me back to the slower profile to fix those issues.
Luckily it was short lived, as they upgraded my area for TV, and with it I got even better Internet service.