Uh, I think I fixed it.

We never changed these settings on the phone base, so I'm not sure why this works.

I changed the EHS mode from GN RHL to DHSG,
and set the bars button to 8/12

whatever that did made the volume and mute buttons work correctly.

The configuration is done on the Jabra's Base.   

I'm surprised you haven't configured a Jabra before!  You mention it on the website as a supported device!  http://www.phonerlite.de/general_en.htm

For the Jabra, there is no software at all.  I'm pretty sure a driver gets picked up though when you first plug in the USB device, but you don't have to install anything, and in fact, there is no software to go with it anyway.

I have a GN9330 here and there are no problems with volume changes. Only after pressing the hook button the call will be terminated. Are you sure that you really don't touch that hook button when using the volume slider?

Can you please do a call and after call is connected clear the debug window PhonerLite by pressing DEL key.
When I change volume on the headset I see events in the debug like the following:
15:41:26,380: HID in: [TEL]: Volume Down = 1
15:41:28,600: HID in: [TEL]: Volume Up = 1
15:41:28,609: HID in: [TEL]: Volume Down = 0
15:41:30,240: HID in: [TEL]: Volume Down = 1
15:41:30,249: HID in: [TEL]: Volume Up = 0
15:41:31,861: HID in: [TEL]: Volume Down = 1
15:41:33,540: HID in: [TEL]: Volume Up = 1
15:41:33,549: HID in: [TEL]: Volume Down = 0
15:41:36,141: HID in: [TEL]: Volume Down = 1
15:41:36,147: HID in: [TEL]: Volume Up = 0
15:41:38,240: HID in: [TEL]: Volume Down = 1

That must be completely different in your case. I am using PhonerLite 2.26.

All right, I figured out a rythmn.
It has to do with the power (hook) button on the headset.

So the headset has two states which can be reached by 
1. terminating a call with the hook button
2. terminating a call using PhonerLite

If you use the hook button to end a call then the headset is in an "off" state and does not consume power.  You can tell it is in this state by breathing into the microphone - you won't hear your breath in the earpiece.

If you terminate a call using PhonerLite, the headset's microphone stays on (you can hear yourself breathing in the earpiece) and the headset will consume power even if you aren't using it.

I like to leave my headset lay on the desk; if I use the first mode, it can stay there all day.  If I use the second mode, I will have to recharge it after a couple hours.

So, regardless of the mode in which you disconnect the headset, you can still make calls normally in Phoner.

However, the volume buttons will disconnect the call if you used mode 1 to disconnect the previous call - that is, if you terminated a call using the hook button.

But if I had terminated the call in mode 2 (that is, by using the red-phone button in PhonerLite) then the volume buttons will work during the next call.

Here's the output from a call that was disconnected by the volume buttons:

09:27:22,643: HID in: [TEL]: Volume Up = 1
09:27:22,644: HID in: [TEL]: Hook = 0
09:27:22,763: Disconnect B3 Request: 0D 00 01 00 84 80 56 39 01 01 01 00 00 
09:27:22,763: Disconnect B3 Request
09:27:22,768: HID out: [LED] OnLine = 0
09:27:22,769: Disconnect B3 Confirm: 12 00 01 00 84 81 56 39 01 01 01 00 00 00 00 00 00 00 
09:27:22,769: Disconnect B3 Confirm
09:27:22,769: Disconnect B3 Indication: 0E 00 01 00 84 82 0E 1F 01 01 01 00 00 00 
09:27:22,769: Disconnect B3 Indication
09:27:22,769: Disconnect B3 Response: 0C 00 01 00 84 83 0E 1F 01 01 01 00 
09:27:22,769: Disconnect B3 Response
09:27:22,769: Disconnect Request: 12 00 01 00 04 80 57 39 01 01 00 00 05 00 00 00 00 00 
09:27:22,769: Disconnect Request
09:27:22,769: Disconnect Confirm: 0E 00 01 00 04 81 57 39 01 01 00 00 00 00 
09:27:22,769: Disconnect Confirm
09:27:22,769: Disconnect Indication: 0E 00 01 00 04 82 0F 1F 01 01 00 00 00 34 
09:27:22,769: Disconnect Indication: 
09:27:22,790: Disconnect Response: 0C 00 01 00 04 83 0F 1F 01 01 00 00 
09:27:22,790: Disconnect Response


However, I have also discovered that you have the same problem if you put the headset on the base and then make a call, and this is regardless of the way you hung up the previous call.

Here is a call disconnected by the volume button after picking the headset up from the base.

09:40:50,775: HID in: [TEL]: Volume Up = 1
09:40:50,778: HID in: [TEL]: Hook = 0
09:40:50,905: Disconnect B3 Request: 0D 00 01 00 84 80 98 42 01 01 01 00 00 
09:40:50,905: Disconnect B3 Request
09:40:50,910: HID out: [LED] OnLine = 0
09:40:50,911: Disconnect B3 Confirm: 12 00 01 00 84 81 98 42 01 01 01 00 00 00 00 00 00 00 
09:40:50,911: Disconnect B3 Confirm
09:40:50,911: Disconnect B3 Indication: 0E 00 01 00 84 82 78 24 01 01 01 00 00 00 
09:40:50,911: Disconnect B3 Indication
09:40:50,911: Disconnect B3 Response: 0C 00 01 00 84 83 78 24 01 01 01 00 
09:40:50,911: Disconnect B3 Response
09:40:50,911: Disconnect Request: 12 00 01 00 04 80 99 42 01 01 00 00 05 00 00 00 00 00 
09:40:50,911: Disconnect Request
09:40:50,911: Disconnect Confirm: 0E 00 01 00 04 81 99 42 01 01 00 00 00 00 
09:40:50,911: Disconnect Confirm
09:40:50,911: Disconnect Indication: 0E 00 01 00 04 82 79 24 01 01 00 00 00 34 
09:40:50,911: Disconnect Indication: 
09:40:50,932: Disconnect Response: 0C 00 01 00 04 83 79 24 01 01 00 00 
09:40:50,932: Disconnect Response

I don't get a hook event when changing volume. Sorry.
I uploaded a new beta version of PhonerLite. Does that change the behavior after call termination? The headset should not be active.