How to stop barking at callers????

I'm still trying to persuade Chip to greet callers in a more polite way. If people come into the house he settles very quickly and treats them like his new best friends. However anyone walking past the house, neighbours in the garden next door or delivery people and he sounds like the hound of the Baskervilles.
When the bell goes he'll run to bed but stand there barking If we're in the front garden and he's off lead he runs towards anyone passing, but then sits and looks cute or comes back to me, so I can't imagine he'd hurt anyone. Its just embarrassing and scary for people who're not sure of dogs. The worst times are when he's on lead on the way out or in and someone appears. I use a down command and put my foot on the lead but this isn't so easy if I have Dale as well. Dale would never think of barking, but once Chip starts he'll join in.

i will be watching this one for good suggestions as werm is the same, its just barking with a waggy tail- but its a racket and scares some clients- how werm could scare anyone is beyond me but,.......
she never barks when on the lead through.