I think a number of companies have the same issue but there does not appear to be an easy solution. some callers will get busy 10 time, some not at all, some once etc etc.
Are you artificially creating the busy tones or is it a capacity issue ?
Either way the easy way to find out is remove the restriction and take the hit. You dont have to route all these calls to agents.
If you are artifically creating busy, use the same triggers to give customers a message.
Try this for one week, count the number of times the message is given. It wont be 100% accurate by any means but it will give you an idea.
Accuracy could be improved by the wording of the message.
Could your system cope with 10,000 extra calls, do you have staff to deal with 2,000 extra calls ?