For a long time I worked in an office with a Sheltie. He was a barker, and not consistently trained by his humans, so the problem persisted. If I was on the phone with someone when he was being particularly barky, I'd say, "Excuse me a second, please" and hush him, then come back to the phone. Or I'd say to the person "Don't mind the noise, that's just the office dog," and 6 times out of ten the person would say "You get to have a dog at the office? You're so lucky," and the other 4 of the ten would say "No problem."