I have quite a bit of experience with PRI T1s in Florida
Who is the carrier?
There are a lot of issues with PRI signalling here in the US because of the lack of standards in signalling.
It is strange that the disconnects are showing up as BUSYs, usually they wil just show as a type 16 normal clearing(which is still wrong).
one other possibility is that these calls are busy congestion on the carrier side, meaning that your carrier can't handle your calling volume. I have seen this happen before.