Solutions at Home
-- Do "No Interrupting" training. Tell your child that you're going to be doing an activity that can't be interrupted (say, talking on the phone). Set your child up with a task that will hold his attention while you're talking, and then take breaks every few minutes to visit with your child and praise him for not interrupting. You also can do a little behavior therapy by using the abacus method, but as part of a reward system.
-- Add incentives. Begin the week with a pot of $5. Assign a value — say 10 cents — to each bead on the abacus or other visual counter. Each time you have to slide a bead because of an interruption, 10 cents should be removed from the pot. At the end of the week, your child gets to keep what's left.
If your child doesn't respond well to the "negative" method, reverse the system to provide positive rewards. Slide a bead for every time your child does not interrupt, to reinforce good behavior. At the end of the week, the child keeps what's been earned.
In both cases, take your child out to purchase a treat with his earned money. The one-on-one attention will cap his sense of achievement and provide additional reinforcement for not interrupting.