I went from "This is she" to "speaking" to "May I ask who is calling?" The last one really seems to confuse things, so I am slowly going to "Who is this, please?" which seems to have better results.
I got a bit tired of admitting it was me only to have them launch into their spiel about 'we've noticed your credit stopped in 2007.. perhaps you had a credit card you no longer use..' (I didn't arrive here until 2009) or some other story trying to get me to sign up for a credit card, or buy something.
If I find out it is a 'junk mail' call, I just say "We're not interested, please take us off your list.", then hang up. Hubby usually says something I'd never say to them, debunking the myth that Brits are far more polite than Americans about these things.