I loved reading a blog entry a bit ago on Dad's Guide to Twins, where Joe wrote about How to Tell Identical Twins Apart on a Phone Call. He offers a lot of advice, including not listening to "the sound of the voice but rather what is being said and how it is said. Pay attention to how your twins sound. Can you tell who is squawking from the other room in the middle of the night? Can you tell who is calling for you from the far end of the house?"
As a sidenote, I always chuckle when I tell people I have boy-girl twins, and they ask if they're identical. My favorite answer, "not when I change their diaper..." Identical twins: One sperm, one egg. Not identical twins: Two sperm, two eggs. Just like any other brother or sister, just happened to gestate together and be born on the same day. More on that...
Back to the voice on the phone. Thank GOODNESS my boy twin is going through puberty, because more than once, I've said "Hi, Alex!" when it was Alyssa, and vice-versa. It's not just identical twins that have the same voice tone.
Joe's advice is good. I've learned to really listen more to the emotion or emphasis of the tone. Alyssa is always cheery and emotive. Alex is usually more casual and cool--he'll say "Hey Dad", where Alyssa will say Hi. Subtle, but helpful.
No matter what, though, I love talking to my kids, whether it's on the phone, on Skype, or most of all in person.