Call was made but not answered
The most common cause of a 0 MOS score call is where an agent makes a call, but the customer they called does not answer or is busy.
In this instance there may be a unique call reference (callContactID etc), but as there is no phone call, there is nothing to measure - so a 0 MOS score is produced.
A call was received but was not answered
If an inbound call is not answered, then a unique call reference (callContactID etc) is generated without a phone call - so a 0 MOS score is produced.
Agents using a Thin Client environment are using the wrong CCP to make calls
A less common issue is where thin client agents have two softphone sessions, one for signalling (thin client) and one local for the media (thick client) - if the agent uses the thin client for media then the call will return a 0 MOS score.