All of the above are probably correct – yet there are many ways in which srvice level can be calculated; dependant on what is agreed by management or even regulators.
ie)
1)service level can be calculated from calls answered in a specific time band in relation to calls offered to the ACD.
2)Service level can also be calculated from your calls answered in your time band in relation to you calls answered in total; in which case:
(Calls Ans in {Time band – ie 30sec}/Calls ans in Total)*100 = Service level