Most IP networks use a best-effort delivery system and treat all IP packets equally. Traffic can experience different amounts of packet delay, loss, or jitter at any time. With data applications this isn’t an issue, and just affects the speed of transmission. With VoIP networks this does become an issue, however, as VoIP is a real-time protocol.
Speech breakup may be perceived by the user as distorted, like the signal on a digital cellular telephone when the user is getting out of range. Speech clipping seems like parts of words are getting cut off. When the user complains about pops and clicks on the line it usually means that there are dropped packets. Echo means that the user hears their own voice reflected back to them from the other end. These symptoms have all been addressed in previous posts.