1. The programming is a very difficult work that needs deep dive into code. Sometimes even small distraction can throw all your work into the trash. Very well it is demonstrated in the comics. That is why calls can greatly reduce productivity. 
2. Calls are very convenient for the clients because they can contact with me anytime. But for me, more convenient is to get emails what I can read in my free time or allotted time.
3. It takes less time to check emails than speaking of the phone. Some clients like to speak by phone, they can speak by phone about their problems for 2 hours! I haven't so much time. For this time I can check 20-30 emails.
4. If client speaks by phone, he usually speaks much and less specifically and I need to take much time to clarify the details. But if he writes an email, he writes specific tasks what I need to do.
5. After long conversation I can't remember everything. But I always can open a correspondence history and read the needed email.
6. The conflicts. The client always can say: "Why didn't you do this task? I said to do it." But maybe he just wanted to say it and he didn't say it. I can't prove it. There are no such conflicts when I use an email because I have a correspondence history. 
7. I can automate mail handling. I can add labels for the different type of letters, autoresponder, group mails by priority, types etc.

These are the basic reasons why I prefer to use an email to communicate with clients. So, please, don't ask me my phone number or Skype, I will not give it to you anyway :)