Originally Posted by chacor

I don't know if it's technically IDB if the reservation was unconfirmed, as you technically need a confirmed reservation to be denied boarding. In any case, if they believe it is IDB, they owe you rebooking (or compensation for your ticket) on their part – not you booking the same flight yourself in the first instance.

I would be going to FCO and asking them for options especially if this is IDB, although as this is an outstation it may not be straightforward; I don't know what IB's presence is in Rome. If they tell you your ticket is simply unconfirmed and not in the system as confirmed, then it can't be IDB – in which case maybe this becomes an issue for AS since they ticketed you.