Agree, to a point - but if a segment is not bookable online, then it does not seem fair to penalize the customer for not booking online.
If the pax tries and gets the "can't complete, call support desk" message and pax calls somewhere else, I'd be more inclined to agree.
If, on the other hand, the message was "neither CO nor its partners serves your destination" when pax knows that is not the case (as has happened to me more than once) that's a different situation.
OP, if you are still reading, what was he sequence of events that led to your making a phone rez?