Personally exchanging pictures does the trick to get it started... sending an unexpected video is always nice to, gets the adrenalin pumping for shear unexpectedness =D

When it comes to the actual phone sex convo, I'd say to just simply vocalize everything you are imagining down to the very T... because you're not only panting a picture in your head. Also try and fiddle around with both partners fantasies, with a fantasy both of your imaginations can run wild. It's also a great way to find out your partners deepest curiosities and or fetishes.

Good Luck, I hope I was of help =D