Vicki
Unfortunately you are going to have to do some digging to really understand this. Depending on the types of calls you are taking (or making), short calls could be quite acceptable, especially in an outbound operation (wrong party contact, no interest, etc).
If this is primarily an inbound operation, it is worth understanding your call types and looking at the call stats at both a global and individual level. Look for patterns or anomalies - high levels of transferred calls, individual agents with unusually high numbers of short calls, particluar inbound numbers with high numbers of short calls - and drill down from there.
If you record calls, identify a group of short calls and see if there are any similarities - if you have access to speech analytics you can do this in volume, but even manually you should get some insight.
Some PBXs and call recording solutions (or call loggers) allow you to see which party terminated the call - it would be worth checking with your Telecoms folk or the manufacturer to see if this is possible in your case.
Like Jonty, I dont think there are any industry averages - most calls centres tend to have different experiences, even within the same industry. Think about what types of calls you take or make and consider what the likely customer expectations would be - maybe a certain level if short calls are acceptable.