Manual dials from the dialpad do override the max attempts set on a queue, however, scheduled call backs do not.
With Manual dials, for example let's say there is 5 max attempts set on a queue to attempt to dial records. If the max attempts have been met, manually calling that records number from the dialpad will override the max attempts set on the queue. A manual call is also considered by the system as an agent selecting one of the most recent numbers they have called, from the agent page (the agent is clicking the number/record from a list of most recent numbers dialled on the left had side of the agent page screen).
Scheduled callbacks do not override max attempts. If the max attempts have been met, for example 5 out of 5 calls have been attempted on a record, then the record will be classed as completed, deeming the scheduled callback as invalid.