I think that the trick is not to sound like all the other call centre drones! Most of the calls I have recieved like this have been from bored sounding, low energy people.
Pick up the attitude! Sound enthusiastic. Be respectful of the prospects time. Ask if it is a good time to talk. Ask them questions about their phone service. I for one, work in the telecoms industy but have no idea how much we are paying per minute... so someone offering fractions of pence savings doesnt really turn me on. However, if i recieved a call from someone who did not sound dead on the phone, I would give them the time to explain everything to me...
Just thought.