Active Matches
When a volunteer is paired to a service user as part of a telephone befriending service this is considered a match, and if you only track one metric then it should be the number of active matches. It is the simplest metric to track to get an indication as to the scale of your service and also provide a simple basis to set service targets and plan resource requirements.
If this is an externally funded project this is likely the absolute minimum that you would be expected to report. It is however very simplistic and for larger projects it is likely you will want more detail.