What do you mean, "reserved seats".
If you are actually talking about reserving just a seat, I would say that it is a little unusual to call German Rail. Most people just get their reservations over there. A reserved seat on a German train at the station in Germany is just €4. In all my travel in Europe in the last 20 years, I think only once was I on a train that was completely occupied (SRO). Even then over half the seats were not reserved.
If you are talking about passholder reservations on night trains, which is actually a supplement for the accommodations, and which do sell out, it's not so unusual and far more sensible than reserving through RailEurope at an inflated price or paying the $50 fee to go through Euraide.