| No Easy answer on Cost per Call.....
I have been involved in well over a dozen medium-large scale helpdesk's and each time the calculation is different depending on the reasons for the calculation.
I took averages across several recognised benchmarks, I came to the conclusion that the CpC averaged at around £10.00 per call.
The official Benchmarking agent I use claims that it's around £8.50 within my peer comparison, other helpdesks of similar services etc.
I personally use cost per contact, as my service make inbound and outbound calls as first contact, + we also use other multimedia options for customers to contact us, i.e. web forms etc.
Hope this helps | 
Linear Mode
