If you charge fixed rates, then you need another strategy than simply demanding to be paid for this.
The strategy should be to make your client find it easier to use Slack or emails instead.
You can't always demand to the client to do XYZ, you can't act as a spoiled dev or someone who adds more anxiety to his day than he already has. He's paying you. No, you don't ditch a client because "he's lazy", there's still lazy people who pay very well or offer great opportunities.
There's also people who-really-don't-care-about-being-charged-more for the privilege of calling you or having you writing their instructions. There's people you work with whose time are worth 5-10 developers per hour; they don't care paying you if you save them time. Make sure this is clear with your clients.
Lazy people are lazy because it works for them. So make it harder to be lazy, that's simple. Or appeal to their laziness, even better! Frankly I don't consider it lazy if a client asks for updates, he's being responsible in fact. I prefer this to being called on Friday night for a job for Monday morning "they forgot" to assign me.
When your client calls, you can let the phone ring sometimes, and then politely contact him through Slack or email a few minutes later and ask how you can help.
Take charge and redirect the communication where you want it.
If they call many times a day, take one call every three call, or always get back to them on your preferred method.
You also have the best reason in the world to require this and redirect the communication in a written way; you need to keep tracks! For practical reasons and legal ones.
When they call, you can also say "alright", listen to them but then ask them: "Can you send me all this through Slack or email to make sure I'm not forgetting anything? Thanks!" After a while, they'll see it's more efficient to write than repeat twice.
Yes, they'll say the "it's easy, do this and that", but keep your ground and tell them it will accelerate the process, that they might get the files done quicker this way. Tell them it really helps you if they send you an email that you can refer to.
If they keep wanting to tell you instructions by phone, then when you answer, tell them you can't take their call at the moment, you will call them back in 2-4 hours but they can write to you in Slack or email if they want to save time and get things done faster. Show you're a busy professional and make them choose to use Slack or email.
The less they'll cooperate, the more time you should add between the moment you answered the phone and said you will call back, and when you'll actually call back. Start with 2-4 hours, then 3-5, then 4-6, etc. It gives them more time to gather their instructions and get prepared (and write them), and they will also discover you're faster when you get emails or written communication.
Give updates; a lot of clients will ask for them because they were not told WHEN they can expect their projects.
So the easiest is to tell them as you reply to their questions, an estimate when you'll deliver the revisions or proofs or whatever. If you take charge on this too, you'll see they'll simply take the habit of waiting for you to keep them updated.
It's normal some of them will verify how the project is going; they're managers! They're used to this and they usually need to do this with their other employees. So be ahead of this and give updates before they ask!
For example, in the morning, contact the client who often call and tell them something like: "I'm working on XYZ, I expect to finish this by the end of the day or early tomorrow morning. I'll keep you updated on how it's going. Have a good day!"
No need to be ultra precise unless you're working on a super urgent project! You don't want to make your clients anxious or they'll call to verify how it's going!
Charge! Very efficient way even if you don't feel comfortable with this. Add it to your invoice, don't mention it and wait until they ask you why you charge them for this.
Then you can explain that you can offer freelance rates because you don't need to hire people or virtual assistant to help you with the calls. Personally, I don't hire virtual assistants for this and explain to my clients that no one could answer their questions on the phone as well as I do, and that I'd rather not make them waste their time explaining stuff to someone else who has no clue about their projects and the job. The easiest to communicate with me by phone or Skype is by appointment... until I can clone myself. If you work with other people in your team, you do the same for them; filter the calls and suggest an appointment.
After sending that first charge for communication and after they asked about it, THEN you tell them that for this time, it's alright, you can remove the charge and let that one pass. You do this once though! Then you charge for real. But give them at least a chance to understand how you prefer to proceed.
As an extra, you can mention that Slack or email communications are usually included in your projects' fees though! In fact you should specify this in your contracts since you charge flat rates, and mention that Skype and phone calls are limited to X minutes and only if something is hard to explain, or at the beginning of the project.