When our daughters were little we had issues with the whole 'excuse me' thing too. It is very tricky because we are trying to teach kids a polite way to interrupt. Interrupting isn't really polite! They need to learn when it is appropriate to interrupt and when it is not. What things are important enough to interrupt for, and what isn't. I had to really work on it. "If there is blood involved, it is ALWAYS important enough to interrupt!" or "If you are hungry or thirsty, it is not important enough to interrupt. You can wait a minute." That sort of thing.
We made a rule that if they needed to interrupt, they should come put their hand on my leg, then wait quietly. This worked wonders! It let me know that they needed me, but still gave me time to finish my sentence before asking what they needed. It worked for in person conversations as well as phone conversations. They still needed to be reminded of what was, and was not, important enough to interrupt, but it took care of the dreaded "Excuse me....EXCUSE me...EXCuuuuuSE ME...!" nonsense. :)
We also had a rule that if they tried to ask a question while I was on the phone, even if they used the hand on leg technique, the answer would always be 'no'. I was very rarely on the phone for more than a few minutes and most requests could wait that long. This stopped their odd need for attention the minute the phone rang. :) Until one day they thought they were clever and started asking things like 'Can I go to bed early?' At that point, I said the answer would never be the one they were hoping for!
Of course as parents we needed to make sure we are not yakking on the phone for ages, or ignoring the hand on our leg for more than a sentence or two, but these techniques worked very well for us, and taught our kids patience and respect for others.