Doorbell barking persists because it appears to work: the dog barks, and the 'intruder' — the delivery person — always leaves. From the dog's point of view, the barking succeeded every single time.

Stop rehearsing it

Every unmanaged repetition strengthens the habit. For a couple of weeks, disconnect the bell or put a note asking people to knock softly, so you can train deliberately rather than react.

Change the meaning of the sound

Record the bell. Play it at a volume so low the dog notices but does not bark, then immediately drop several treats. Repeat until the sound makes the dog look at you expectantly. Raise the volume in small steps over days.

Give an alternative job

Teach 'go to mat' separately with good rewards, then ask for it when the bell rings. A dog cannot bark at the door and lie on a mat at the same time.

What not to do

Shouting reads as joining in. Anti-bark collars suppress the noise without touching the underlying arousal, and often produce a more anxious dog.