[VoiceOps] fraud protection

Hello, We just had an unfortunate compromise and racked up a large amount of calls in a 12 hour period. The attack seems to be for financial gain in that the most frequent destination is a conference call service in Poland, that possibly keeps calls open waiting for a PIN to be entered. Is there any basis for expecting that the upstream carrier should have some protections that would limit our liability? Thanks, -mark P.S. For those people who feel compelled to point out that we should have (better) protection on our end: Yes, Thank you, message received!