Final outcome of the technology is in the last section of the post
Currently, the cell phone seems to be becoming the "do-everything" tool that the public uses for all its mobile personal needs when it comes to needing or storing information. (Assumably you call someone because you want to send or receive information?)
When they first came out, they were large and could only place calls. Since then, they have gotten smaller and smaller. They had more and more functionality put into them so they became the only electronic device you need when you are on the go, as well as became a personal storage device (Don't see people with briefcases as often, except where paper copies are required).
Then, they started getting bigger again. Why?
Touch interface is easy to intuitively grasp, but requires adequate screen-space.
Having a larger screen is more visually attractive as clarity and definition increases.
It doesn't make the phones any less mobile.
It seems clear that having an easy-to-see visual screen is of some importance to the mobile electronic helper, as well as having a simple and easy to use interface, otherwise it would have continued to get as small as possible while being able to fit major functions.
To expand that to your situation - you have a person in space, probably in a suit of some kind? He at least requires some way to breathe. This suit or headpiece, or whatever the largest object you have that is personally owned and has to go everywhere with you, is probably the do-it-all device which is also capable of providing a mic of some kind near the mouth, and speakers somewhere in the vicinity of the head.
Imagine what this person will typically be doing while placing a call. If his hands or eyes cannot be distracted, he probably has to use his voice or some other way without having to look and push a button, though a button or physically interaction would probably still be available.
Then consider the screen aspect - how can they view images with their device. Holograms could be projected. If they wear a helmet, it could be a screen overlay on the interior in front of the face. It could be a touch-screen built on top of their fore-arm.
Depending on the technologies you want available, the solution you choose may heavily depend on a person's daily routine. If you drive to work everyday in your space jumpsuit, its likely going to be built into the jumpsuit. If you just walk down the hall, it may be an object that automatically follows you around.
My ultimate solution
If just about any technology is available without adverse side-affects, I would think the perfect solution is something that is able to always be with me, but not directly implanted. It could continuously scan my brain for commands and do them just by me thinking them - with sophisticated enough technology in that area it would literally know exactly what I'm thinking. It would for the most part require no charging or plugging in on my behalf. It could easily provide audio for only me to hear, and can easily provide clear visuals. The smaller it could be, while being able to provide the above, the better.
I am thinking something the size of a coin, which is typically carried in a simple slot on a belt or in a pocket, and has the capability to automatically follow you by some type of floating or propulsion technology. It is linked to my brainwaves to know when I give it a command. If I want to place a call, It comes up to my ear to provide audio and I can either talk normally or provide brain-commands. When receiving a call, it will notify me in my preferred method. Could be an audible noise, a slight visual, or vibrating. It can project an image in front of my eyes, either a translucent one or a completely opaque and super-ultra-HD one. When I get in my spaceship, there is a spot for it near my head. If I put on a helmet, it goes into the helmet first, automatically by brain command. If I am a big-shot CEO or something, maybe I have many working in conjunction - simultaneously calling people with messages, driving me to work, and grabbing my coffee for when I get there, while I use another to read the morning paper.
It would seem like an extension of yourself, without actually being one - meaning you could assign it simple tasks and not have to worry about the little details of getting the task done. You can easily get it replaced if it malfunctions, send it physically to other places if needed, and store highly sensitive or personal information only on it.