The general rule for a call dropping in this manner is that the call was "rejected" either in the switch or the CO. It could be an Switch issue (no path found), or the CO (missing or incomplete digits). In either case it appears to be a "presentation" issue if the call succeeds after failing.
Open a ticket with the carrier and have them do a "D" channel trace while you make test calls to see where the failure occurs.
I suspect it is a CO issue, because the PBX will generally give a "turkey tone" if it is a switch issue.
Depending on the Serving CO switch type and config. you may get a "turkey" or just a simple "drop" which clears the call from the station calling loop button when the call fails.
It could also be that the "pipe" is taking some major errors, as TTT and SY suggested, investigate your switch first.
If corruption is present (errors) on the "D" channel messaging it will result in a "reject" also.