My wife was on the phone calling to get some info about a project we wanted done.
The guy on the phone said, "Hold on a second."
She could hear some rustling around, heard him call another person's name. He finally came back with an answer that she wasn't sure was directed at her or somebody else. She asked for clarification and he said, "Sorry about that, I'm doing three things at once."
Ummm... Pick one.
Pick one, do it well, do it completely. Don't answer the phone if you cannot give that person your full attention.
My guess is that he didn't upset one customer. He probably upset all three.
PS If you have to answer the phone because you don't have an answering service, first ask permission from the current customer. "Excuse me, may I answer this call?" If the customer gives you permission, then answer it, get the info and promise to call them back when you're done with the current customer. A little courtesy goes a long way.