(a.k.a. silent calls, nuisance calls)
An ‘abandoned call’ is said to occur when a consumer answers a call but no agent is available, so the call is ended.
Abandoned calls are the inevitable waste product of predictive dialling, where more calls are launched than there are waiting agents, in anticipation of some of them not being answered, some being answering machines, etc.
Abandoned calls are inextricably linked to predictive dialler performance. This has led many users into bad practices. In order to minimise the time the agents spend waiting for a call, users simply increase the dialing rate (initiating more calls per waiting agent), therefore increasing the abandoned call rate.
Largely because of this effect, in 2003 Ofcom introduced regulations that limit the number of abandoned calls allowed to 3%. In the years running up to this regulation, abandoned calls were many times this level.
For details of the regulations, see the full text from Ofcom (PDF, 180Kb).