I’ve had the same issue. It seems to be that it takes too long to switch to 2g and the call goes to voicemail, as it’s usually fine if already on 2g.
There is part of me that wonders if it’s partly spending time looking for 3g. However there is no setting to only use 2g or 4g.
Agreed, and the carriers should be forced to accept any compatible device.
AT&T and Verizon do have whitelists, while T-mobile technically doesn’t; they are fairly open to any device as long as it has been tested and certified for their network. I suppose you could also call that “whitelisting,” in the same way as FCC or EC certification “whitelists” electronic devices for market.
At any rate, that is just how I understand it as an unqualified layman who has done some reading on the internet. 
I don’t think so, I set manually the network mode to 2G-4G but it behaves the same
mmcli -m 0 --set-allowed-modes='2g|4g' --set-preferred-mode='4g'
(note: check the modem number with mmcli -L and adjust the command accordingly)
Nope, the caller doesn’t hang up, the call goes to voicemail … I don’t know exactly what is the issue, I guess it takes too long to make the switch, but I’m not 100% sure it’s able to downgrade automatically to 2G. I wouldn’t know how to find out.
For a data-only SIM solution that bypasses local carrier device restrictions, I suppose that an international travel SIM might work, although most of them are expensive to use.
I did find this one, though: Surfroam, HQ’ed in Estonia, which has a data-only travel SIM that seems quite reasonable for many countries (including €0.01 per megabyte in Europe, €0.02 in the U.S and Australia). That’s not too bad, especially for low data usage. And it can be kept active indefinitely by recharging at least every 365 days.
In any case, sounds great for actual travel abroad. 
[Edit: I just ordered a Surfroam SIM, so I’ll test it and report on how well it works (in the U.S., since I’m not traveling overseas any time soon.]
ICYMI: Using Phones with No-SIM Cards and Other Privacy Experiments
While waiting for the Librem 5 USA, I purchased one of Braxman’s De-Googled Pixel 4a phones running LineageOS 18.1 which has been working great except for SMS via ting.com on T-Mobile so I’m seriously considering a non-SIM card option as described above or abandoning SMS altogether.
I have 3 deGoogled Pixels( 2 with Graphene and 1 with Lineage. They both work very well on AT&T
My data-only Surfroam SIM arrived from Estonia (to California) in just 4 days. I initially had no success getting it to register with AT&T. The Surfroam team helped me over chat and I eventually got a strong LTE signal. This is on an Android phone.
I successfully added a SIP account to the Android with Linphone as the client (native Android dialing client probably works, too), as an extension of my home VOIP phone, and it works well. I can set it to either ring both my home phone and the Android simultaneously, or let my home phone ring a few times, then ring the SIP extension.
You could also get a VOIP provider for a dedicated mobile account only (with no need for a home VOIP phone), I’m sure. Some providers even have some basic SMS functionality.
So, I think that with a data-only SIM and a SIP account, this type of setup would allow anyone to use the L5 without VoLTE as an interim solution, once SIP functionality is added to the dialer.
P.S. Surfroam works in most countries, and the cost per MB is fairly reasonable, compared to other travel SIMs.
EDIT: These tricks might be useful, too: https://20somethingfinance.com/how-to-send-text-messages-sms-via-email-for-free/
And:
https://avtech.com/articles/138/list-of-email-to-sms-addresses/#t
So, SIP calling is already available with other clients including twinkle, jami, linphone, etc. This video is over a year old but shows using jmp.chat with the Librem 5. This one was using XMPP, not SIP. Note that it was possible to get text messages as well. This is one of the issues with using VOIP, will get you be able to send/received SMS and MMS? More and more of the SIP VOIP providers are providing SMS support, but not so many support MMS it seems. jmp.chat supports MMS as well as SMS (based on what I read), but note AFAIK jmp.chat currently only supports getting US and Canadian phone numbers.
Another thing I wonder about is any difference in privacy and security with VOIP options (SIP and XMPP) versus a SIM card plan with voice/text/data. I guess once you are on the PSTN (the regular telephone network) your risk is the same with regards to capturing information on the PSTN, but I wonder if you use a smaller provider like jmp.chat or Zadarma is your metadata less likely to be sold or given out than when dealing with AT&T, Verizon etc. With respect to SMS and MMS, since that is sent sort of out-of-band (at least on VOIP from what I have read), does that mean the only way intercept those is from your carrier or VOIP provider?
Note also, in addition to Surfroam (mentioned earlier), I noticed (in searching) that also Keepgo also offers similar types of data-only SIMs.
I wish we had an idea of what the probabilities of VoLTE for Librem 5 on the various carriers in the US was. I may or may not be willing to change carriers (to one support Librem 5 VOLTE) depending on various factors. At this point, I am not sure if there are real advantages to traditional carrier with VoLTE vs a VOIP based approach, if one can still send/receive SMS/MMS (some services may not send a verification SMS if they think the number is not a true mobile number).
The other thing I wonder is whether maybe it would have been a better idea for Purism to just come out with a Linux computer that could be used as a phone (Librem 5) solely for a data SIM and VOIP client support, maybe even partnering with one or more VOIP providers. At this point, with the right VOIP provider (like jmp.chat) it appears I would at least have MMS support at this point and not be worrying about my carrier dropping support in 6 months. And with data-only SIMs like Surfroam or Keepgo I could theoretically travel the world (not have to change SIMs in different countries) and just configure multiple phone numbers. The documentation for twinkle says it supports two different lines. Other clients may do similar. So I could have one number in the US and another in Europe for example on the same phone.
Jmp.chat does do MMS. A picture, for example, shows up as a download link. You use xmpp to send SMS and MMS messages to contacts formatted like xxx-xxx-xxxx@cheogram.com, but texting the jmp number is just like texting any other phone number. I don’t know about international numbers nor how group messaging works as I’ve never thought to try.
NOTICE: It now seems that Surfroam is changing their SIM card technology, and even existing SIM customers will have to purchase and ship a new SIM in order to keep any remaining balance from the old SIM after March 1, 2022. (I guess that was a wasted purchase of €47 for me (€25+€22).
Caveat emptor.
From Surfroam’s website:
Will my Surfroam SIM expire?
Your Surfroam SIM card initially valid for one year from the purchase date.
Every time you top-up your Surfroam SIM balance, it is prolonged to next year.
However, in case the year passes without you having used your Surfroam SIM card, we reserve the right to expire the SIM and reset the balance without any warning.
I’m confused. What does this have to do with VoLTE? Does the new SIM only allow 4G or something?
No, it’s just that we were talking about interim data-only options up the thread a ways. I had mentioned that I was testing a Surfroam data-only travel SIM (which runs on AT&T when in the U.S.) as a potential solution until VoLTE is operational.
Because U.S. networks won’t activate a non-VoLTE phone or allow a data-only SIM to be used in a phone.* A data-only travel SIM can skirt those rules. (And adding a SIP account allows phone calls and texting with the data SIM.)
(I don’t know how Surfroam’s new SIM is different from the old one.)
*Not an issue in some other countries.
This makes me very sad. I was wanting to do this very thing.
You never know… The carrier might think the L5 is a car or a laptop. Try it!
Well I was wanting to do so with my pixel in the meantime. Alas…
A (hopefully cheap-rate) international travel SIM will probably work with the Pixel. Surfroam’s data SIM works with my Android, with no complaints from the underlying roam network.
Bonus: The roam network may not get much PII on you from the overseas company. Plus, court orders might be harder to serve in a foreign country.
Keepgo.com is alternative to surfroam for a international data-only sim. Disclaimer: I haven’t tried this yet. I am considering it for the future as it appears that having VoLTE work on the librem 5 seems to be mainly up to the mobile network providers, not Purism. For the networks with a whitelist, if the librem 5 isn’t on it, there is nothing Purism can do. It is not really a technical issue at that point.
Per a response from @Kyle_Rankin on another thread (seen here New Post: I (Finally) Fired Google ), VoLTE can work with the BM818 (although Kyle was only saying it worked with his Librem 5 with AweSIM service). I don’t really want to spend $100/month for a phone that I mostly only need cellular service for phone calls (as I’m within trusted wifi range 95% of my life), so I’m attempting to get VoLTE working with service from T-Mobile. 4G data works just fine, but calls are only working via 3G (which I believe T-Mobile is planning to shut down in July). My IMEI shows up as fully compatible and not blocked on T-Mobile’s IMEI checker. Does anybody have experience with getting VoLTE to work with T-Mobile or a MVNO running on T-Mobile’s backbone?