You could run the following command:
list measurements trunk-group summary yesterday-peak
On the second to the last column the %ATB means percent all trunks busy.... which would translate into a busy signal to the caller... this report would give you yesterdays peak....
As a dirty historical method, you could use ASA to run this report a few minutes past every hour to get data on a whole day...
On the other hand if you had CMS and trunks measured and defined, that would be easier. Run a historical report on the trunk.
Lastly, if the trunk only handles toll free numbers... the carrier of the toll free nubmer would be able to tell you how many busy signals were presented when called.
Thanks,
Wildcard