I find this situation incredibly irritating. I ring a colleage, but get through to their answer machine, at which point it hang up immediately. However, i always get that annoying reminder, telling me that i have just used one minute from my goodybag.
It wouldn't be so bad if there were something in between 300 minutes and unlimited, but as it is i have to be very careful with my minutes.
Surely it wouldn't be difficult to charge for any call less than 5 seconds? We're not talking free calls here, just that couple of seconds it takes to put the phone down when one realises you're not going to get through.