ADHDer's are ALWAYS interrupting people. It's an extremely common, albeit annoying, symptom of ADHD. There are many things coming into play at the same time – impulsiveness, difficulty with working memory, boredom, enthusiasm, quick-thinking (rightly or wrongly anticipating what the other person is going to say).
So what's the parent (or spouse) of a child with ADHD to do? We don't want to be correcting them ALL the time, but we want to finish a sentence!
Here's a little structure that gets great results. Teach your family and make it a family routine.
When you're having a conversation, whether it's in person or on the phone, when your child wants your attention, she can put a hand on your arm so you'll know your attention is requested. Then, you can put your hand on your child's, so she'll know that you're aware she wants you. Your hand on hers means, “I will get to you in just a minute. Please wait.”
Then, as quickly as you can, ask to be excused from your conversation and give your attention to your child for a moment. Don't deal with her issue at that time, necessarily – you don't want to teach her that she can stop you whenever you're talking. But once she's been heard, she'll likely have an easier time waiting until you can get back to her!