This is a read-only integer, in dBm, recorded in the call history table showing the average background noise level measured over the duration of a now-completed call leg. An admin uses this historical average, as opposed to the active-call object's instantaneous reading, to assess whether a completed call had a persistent noise problem rather than just a momentary blip. For example, an admin reviewing a batch of completed calls on a suspect line could look for consistently elevated average noise levels here as evidence of a chronic line issue.