Using call waiting feature

[I made a mistake and posted the question in the wrong forum. Could you move this to 'Ask a Teacher' forum?]

I was on the phone with a rep from The Brick, an appliance mall, because I bought a fridge days ago and now it leaks water. As I was describing the problem I heard the 'beeps', indicating I have an imcoming phone call. So I asked the rep to hold for a moment, pressed 'flash' button on the phone panel. Another person on the phone. He's looking for my dad.

I had to tell the person to call later so that I can finish business with the rep. I told him "I'm in the middle of a phone call, can you call back in about... three minutes?". He said ok, and hung up. Then I pressed "flash" button again to switch the line back to the rep. I continued on from where I left off, arranged for a technician, and hung up.

This got me thinking a while. There must be better ways to say when telling people I'm on the phone with somebody else.

After a while I came up with a couple more.

- I'm currently on line with someone, can you call back later?
- I'm on the phone with somebody, can you call back later?

What would be the conventional, English / American / Canadian way of saying in such situation? What would you say?
Oh and if you can, can you correct any errors on this post? (Would you proofread?)

Your feedback is highly appreciated.