Make sure you’ve dialed the right number and you’ve reached the right person. : )
Make sure the person you are “conversing” with is completely alone and 100% comitted to talking to you in this manner (it can be fun, but also a little bit intimidating)
Make sure your partner is assured that you are alone, not recording him/her and are really interested in doing this as a form of mutual passion (and not just for fun or a kinky kick)
Be aware, that at the moment of greatest passion, one or both parties may need to put down the phone, so discuss that ahead of time so that no one thinks that the other person has hung up, died, or gone to the bathroom to laugh or engage in more “immediate” relations.
I have no direct experience with skype, but that may be a better way to go, as long as both parties are assured that no one is taping the encounter. Most women are now afraid of the “Charlie Sheen” effect.