but is 15-20 minutes so unusual for a US call center?

Depends on what the call center's purpose is.

If the purpose is to sell products, then most company presidents would turn white as a sheet if they discovered that their customers had to wait 20 minutes to make a purchase. Imagine how many people would hang up the telephone and dial their competitor after the first five minutes.

If the purpose is after-sales support, then no, 15-20 minutes is not unheard of.

For support, I don't mind waiting that long, provided that the end result of the call is a positive one. If I waited that long but didn't have my problem resolved, then I'd start to get upset.

What makes me really angry is telephone systems which make me punch through twenty levels of menus before I can reach a human being. Three levels of button-pushing, tops, it what I consider tolerable. I've reached the point where, with most places I call, I pound on the "0" key repeatedly until an operator answers, then I tell the operator where to send me. If that doesn't work, sometimes just staying on the line without pressing any buttons is enough to drop to the switchboard operator. This might sound rude and selfish, but there are many places where that's the only way I've been able to reach my desired destination without getting disconnected.

And that's what I get the MOST angry about: When I do punch through several levels of menus, only to get disconnected because of a poorly configured telephone system. This has been happening to me with increasing frequency. In the past year, I think about one in three times I've called a major vendor looking for help, I've been disconnected during the call. That is the most inexcusable thing I can imagine.