Instead of putting people on hold in this technological age, can't we put them in a call-back queue? Then they can hang up and go about their business until someone is ready to assist them. This would be particularly helpful when congestion at the call centre is very high, but would actually be bad when it's very low, since it would be more trouble in that situation. I also imagine people would be very distrusting of such a call-back system, and would rather wait than risk never hearing back again. Still, if you trusted it, all you'd have to do is call the queue line, state your name and reason for calling, then hang up. Caller ID can handle the number, or you can specify another by using your keypad. If you're on a mobile, you could even text in a call-back request.
Mokalus of Borg
PS - This doesn't even seem like it would have to be very high-tech.
PPS - Would you trust a high-tech version more than a low-tech one?