I have used signal through a virtual machine before, so there is no need for an actual smartphone to use signal. The problem I ran into was that my android virtual machine couldn’t access the camera to scan the QR code for linking the desktop app. So I haven’t been able to use the desktop app without re-linking through a smartphone.
What is the real goal here? Are you trying to accomplish some technical achievement or are you wanting certain features that can only be obtained by using GSM, 2G, 3G, 4G, LTE services? With a WiFi card and any VOIP software, you have a PC based phone with no monthly charges. If you want a phone number to go with it, you can use Google Voice. Verizon can sell you a laptop that has a 4G LTE modem built in to it. But you’ll have a monthly bill that way. Exactly what features (not methods) are you looking for?
Mmm but does it work for audio (voice) calls or only for data calls?
Based on the first post: The goal is a single computer, being the OP’s existing desktop computer, that just does everything.
That will allow the OP to get rid of the Android phone, presumably a privacy win, and potentially a cost win.
My assumption is that the OP has no need for portability / mobility at all and, despite living in a rural area, no need for a mobile phone for use in emergency situations. In other words, I wouldn’t do it - but the very fact that this is hard makes it a worthwhile challenge. 
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I want to own as few separate devices as possible.
I want to avoid running costs as much as possible. (One-time expenses are less important.)
I want to avoid non-free/closed source solutions as much as possible.
I want to be sure I can easily communicate with anybody, especially medical/banking/police and such authorities in Finland who may use old and/or proprietary solutions.
Mobility of the chosen device is not important to me, but it has to work without a wired internet connection.
Another “project” I have is connecting a trail camera to my own WiFi network and have it send video to my own hard drive, without involving cloud services. I spend most of my time on a small, isolated wooded island. If I have everything concentrated in one place, it’s easier to make backups of both data and the system itself. For emergencies when out at sea and such, I can have a separate rugged “dumb” phone in my pocket.
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I believe the reason this is made difficult is the potential for saving money in the long run. Privacy and personal control is a given, but I would also rather spend 100 € once, than 1 € a hundred times (or more).
You could use Anbox to host the Signal app…
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So I figured I would come back to this, since I made a breakthrough today in my personal communications journey.
I was able to successfully link between the Signal app–installed on a Android Virtual Machine (using VirtualBox and Android x86 image)–and the Signal desktop app installed on my Linux laptop.
Basically, using only official Signal builds, I got the Signal desktop app working on my Linux machine with no physical phone involved!
I was running into a couple issues, so maybe this will be helpful for someone else interested in doing this:
The main problem is that I couldn’t use the camera in the VM, because the camera is required to scan the QR code for linkage to the desktop client. To fix this, I had to add myself to the vmusers permission group. Then, I had to purchase an independent webcam, as it seems like the signal app expects front and back cameras, so if they aren’t both present, the app crashes when trying to scan QR code.
After all my setup was complete, I was able to bring up the VM and also the desktop client, and then link them–all on the same machine!
Anyone can feel free to PM me if they want to do this and have questions.
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Even better would be if you wrote up a step-by-step tutorial and published it somewhere, but I understand that is a time commitment.
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I think if anyone (yourself included) tells me that they want to do it, I could write a tutorial. But I know I am pretty idiosyncratic (as many here are, I’m sure).
Most people probably just use smartphones or not use signal, but I would be happy to put together a tutorial if anyone would benefit.
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I think a tutorial could benefit many even if they’re just looking for options and not 100% serious about using Signal without a mobile device somewhere in the setup IMO (of course whether thats worth your writeup time is another matter entirely )
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At least you’re not alone :).
A tutorial would be great.
Which phone number did you use to register with signal? Did you try to use throw-away numbers or fixed line numbers?
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I suppose you could get Signal on the L5 through anbox, and then just manually enter the verification SMS. I tried it yesterday through AMUDOS which is still on Lollipop and it worked, so there is no reason why it wouldn’t work on the L5 with anbox.
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Haha I figured
Alright, I’ll write something up and try to take some screenshots so people can follow it.
Edit:
@ChriChri Sorry, I forgot to respond to this. I just used my google voice number to register for Signal within the Android virtual machine.
Doing it all on the Librem 5 can probably be done, but with another machine (desktop or laptop), I am fairly confident the Signal desktop client would work on the Librem 5. I could just install the Android Signal app on the virtual machine on my laptop or desktop, then use the webcam to scan the QR code on the Librem 5 linking it to my virtual android phone.
I’ve never used anbox much, but I would be inclined to think that the Signal desktop app would perform much better than the Signal android app running with anbox.
Of course, that’s just a mostly uninformed guess until someone tests them out.
Also, I recognize that the desktop client does not yet implement all features, so that could be a factor in deciding to use anbox. Probably the biggest shortcoming right now is no creating new groups or managing groups.
Hm, is that ‘known’ as a mobile or as a fixed line number? I wonder whether one can register with any number…
I can receive SMS text messages on the google voice number, so I would guess that it is considered a mobile number, but I don’t actually know.
It is a mobile number.
That means you need to have a primary client somewhere however.
Google voice has a method through it’s Legacy Settings panel, to tell Google whether any given phone number is an old fashioned landline, or a mobile number. Any number that you port in to Google Voice will always be a mobile number if it wasn’t already a mobile number to begin with. The only reason that GV has the ability to recognize non-mobile numbers at all is out of convenience to the user. You wouldn’t want to expect that your text messages should be sent to your landline when forwarding calls. So GV greys-out the text messaging option to landlines on phones that you forward calls to. When you add a new phone number to forward to in GV, the default is usually for GV to assume that it is a landline. I recently added a new cell phone to forward to GV and had to figure out how to tell the interface that the new number is to a mobile phone and not the landline GV assumed it was, by default. GV allows you to own many phone numbers under them as you want. But you have to pay a one-time $20 fee for each one.
I have noticed that some banks have stopped allowing GV numbers for second factor authentication.