If a voice only needs 8 kilobits or 15 kilobits of data, that s all that gets taken out of the pool. All the rest of the bandwidth is available for others to use. But when the bandwidth is scarce, that s when you need to have very deterministic QoS models. These are what people would refer to as guaranteed. It s guaranteed from the point of view that the bandwidth is allocated for the call and it remains in place for the duration of the call.
Nobody can take it away.
The most prevalent one, is that they don t do the traffic engineering quite right, so you can make a network design error. You forget how many you ve already got on the network. You can configure things wrong. Other types of mistakes are a little more operational, not so much design. It s easy to make mistakes.