[Scoundreller]

> they have to place a second outgoing call which may have different charge rates than the incoming call did.

It's probably a wash: their outgoing call to you costs as much or less per minute than their toll-free incoming lines. At least in North America where Calling-Party-Pays isn't really a thing.

In cellular plans that don't include long distance calling, it could cost you more to receive a call when you're out of your home area, while the same toll-free call is (usually) never long distance.

> somewhat common feature of customer service phone systems to have a way to do callbacks

Maybe on the sales side, but on the service side, many vendors (particularly b2c) prefer that you give up entirely and never call them ever again.