In a properly tuned network, the maximum delay between the time a calling party speaks and the called party hears what was said is less than what the average person can detect. In other words, there is no perceptible delay in the conversation - words are heard as soon as they are spoken. In some cases delay across the network will increase to a level that is detectable by the human ear. In a worst case scenario, the conversation will deteriorate to the point where each person has to indicate that they have stopped speaking so that the other person will know that they can talk without being interrupted by voice packets that have not yet been received. If you are familiar with Ethernet you might call this behavior a Late Collision.
All networks impose some delay in the transmission of packets from source to destination.
Variations in delay are caused by the same situations that lead to long inter packet gaps as described in the section on jitter above. In the case of delay, however, the causes typically last longer than a few voice packets. Instead of causing an increase in the inter packet gap for a few packets in a conversation, these problems affect entire conversations. The 79xx doesn't provide a delay counter. Delay can be measured by network analyzers and other network management devices.