Trying to teach your child not to interrupt can sometimes be an exercise in frustration.
Telling them there's a time to interrupt (in case of a fire) and a time to not interrupt (boredom) isn't enough. But putting these principles into practice is easier said than done, especially for a very verbal or high-energy kid. That's why now is a good time to revisit some basic lessons about good manners and teaching your child to wait their turn to speak.
First of all, set a reasonable expectation. School-aged children have a difficult time holding their thoughts for more than a few minutes. Indicate to her as best as you can that you'll be with them as soon as possible and then stay true to your word.
Develop some ideas for them to occupy themselves with while you're on the phone or otherwise unavailable. Keep a box full of puzzles, crayons, colorful markers or other quiet toys nearby that they can only use when you have to make a call. Set snacks and drinks on an accessible level so they don't have to interrupt you for help.
When you need to make a call or have an important conversation with a visitor, head off trouble by saying you're about to phone someone or have a conversation and estimate how long you expect to talk. Ask them if they need anything before you make your call or have your conversation with your company. Then do your best to adhere to that time schedule, and excuse yourself from the conversation long enough to check on them. Let them know you'll be a bit longer if that's the case and see if they need anything before returning to your conversation.
Reading is a great tool to teach manners. Find several books on the subject then read them together. Discuss afterwards what your child learned from the story and how they'll handle a similar situation in their life the next time it occurs.
And as always, children learn what they live. Your child is very unlikely to learn not to interrupt if they hears you, your spouse, or their siblings constantly interrupting each other. Your actions have a strong influence on your child, so be a good example and ask permission to speak before speaking, and apologize when you inadvertently interrupt.