You can get a "phone number" to have your V4 call its parent, but there are no phone numbers for someone on the friends list. (Phone numbers are listed in this topic: https://www.tamatalk.com/IB/index.php?showtopic=93939 )
However, if you connect two V4 Tamagotchis using the visit option, and both have the phone, they will sometimes appear to be talking on the phone to each other. Here's a photo spudilike took, of this happening: https://img227.imageshack.us/img227/1847/phonevisitzo0.jpg
No, you can't "talk" to people on your friends list, however you can "connect" and even play games and visit!
You can call your parents if you have the cell phone, and you can give gifts to your friends when you "connect" but Tamagotchi's cannot talk to eachother.