Just to get back on topic, I think that we have to look at the function of the communicators at the time. Basically, they were 23rd century walkie talkies. In this case, there were capable of reaching spacecraft in orbit (I can't recall an episode where the range exceeded a planetary orbit, but there may have been a case where it did). It also served as a link to upload data from the tricorder to the ship.
Could the tricorder connect with the ship independantly of a tricorder? Again, I don't recall it happening, but anything is possible.
Having said that, you figure that you have a speaker/microphone, frequency alternator, volume adjust and an function to allow the moire pattern to also act as a video screen. This function was never used on screen, but it is certainly within the real of possibilities, since it was already a video screen that showed a moire pattern. It couldn't be difficult to use the screen for other functions.
The other thing is to allow the buttons to change functions (similar to organic LED, OLED, technology).
My $0.02.
That's pretty much what I was talking about, too.
The only major tweak I'd made would be to alter all five "little round do-dads" to be multi-functional elements. Each would have a multi-color light element in the center (to serve as an indicator). Each would be "clickable" (think like a mouse-button). Each would be a "jog-dial" type device (rather than a simple rotary knob... meaning that you control the "panning speed" of its associated function by deflecting it more and more... fine-adjustment is a small turn either way, quick adjustment is a larger turn either way.
You'd start turning one of these and information about whatever you were doing would show up on the little round screen (adjusting to a different frequency-set, adjusting security settings, adjusting speaker volume, adjusting microphone sensitivity, adjusting operational mode... that's five, by the way, and are the five I'd use!) Once you get the setting you want, you "click" the button and that accepts your change.
The bottom two would be "frequency-set" and "speaker volume," the most commonly-used.
Why "frequency-set" rather than "frequency?" Because I still imagine this collecting your speech (or the audio-visual composite data if you were in that mode) and then, periodically, burst-transmitting it (along with a "key" to tell the next frequency in the series, in some encoded form). This would make the device far harder to trace. Every burst transmission would be milliseconds long (or less) and would be on a different frequency. But you'd need a "starting protocol" to use in order to be able to initiate communications... you'd have a list of those protocols, and you'd select that by this dial.
Note that this "jog-dial/selection" mechanism requires visual feedback. That would be shown on-screen.. probably as a rotary selection diagram (think "pie chart" if you need to visualize something), in some cases, and as a simple text line in others (say.. "FREQ1" or "FREQ2" or so forth). But however it's done, you need visual feedback in order for this to work... thus you need the round thing to be a screen.
"Security settings" might be combined with the "mode" function in some fashion. However... I like this as the center jog-dial in the upper row of three, because that's the one that we saw, when depressed by a hot Yang babe, broadcasting an emergency signal.
So... in that case, if you clicked and held it for several seconds, it would send the beacon signal and flash red. But you'd normally only "click" those buttons... and dialing it left or right (as described above) and clicking would allow you to go to more secure or less secure preprogrammed settings.
Input sensitivity would also be a series of preprogrammed steps... probably a different set depending on whether the device was serving audio-only or audio-with-video modes. If video was there, you'd have more choices, because you'd need to compensate not only for ambient noise but also for lighting conditions. (You might also have a programmed mode that would allow it to see in the non-visible portions of the spectrum.) I'd probably make this the upper right-most jog-dial.
The upper left, then, would be a "mode control" which would allow you to select the operating mode. Modes I can imagine being present would include:
1) Audio-only mode (less power consumption, more secure, longer battery life than A/V mode)
2) Audio-visual mode (useful for diplomatic situations or "friendly" situations, mainly)
3) Recording mode (useful to, for instance, record "Captain's Logs" while off the ship).
4) Paired-operation mode (with a tricorder, for instance)
5) Programming mode (which would significantly alter the function of all of the other dials to make for a semi-practical field-configuration tool. Normally, configuration would be performed on the ship, by an external device).
6) Standby mode (minimal power consumption but still monitoring for any specific transmissions to the device)
7) Stowed mode (for charging and external programming/control by a base-station)
That's how I envision it working. Feel free to toss your own thoughts into the mix, of course.
Now if I could just get people to stop pronouncing it "PeeTrope" and capitalizing the "t". 
