Andrew
No one can accurately predict abandonment rate as there are too many factors involved. How long would you wait for a sales line, for a claims line, on a freephone line, staff available when you call, sudden surges in calls etc
However if you do want to try, there are free calculators on the internet where you can specifiy the time that callers will wait before giving up. you can then for any given call volume and handle time, estimate the number of abandoned calls.
Abandons will vary depending on the business you are in, the cost of the call and how important it is to the customer that they speak to you, the ease of alternatives, e-mail, quality of your queue messages/music.
If I need your help, I will wait for a long time, if I am buying from you and you dont answer, I probably wont wait for so long.
Standard Erlang assumes callers will wait for ever but there are versions out there that will estimate abandon rates for you if you really need it.
You could use look up table based on your historical data. IT may be that 100 call generates 2 abandoned calls,125 generates 15 etc etc