Stupid question I know, but after 5 minutes looking for a relevant recent thread I got tired.
L5 arrived just recently (yay, awesome) but now I need to take care of the carrier sign-up question. I recall seeing a thread or two about some inexpensive US-based carriers but I couldn’t find them. Any recommendations for US east coast carriers?
Which of the major networks has suitable coverage where you spend most of your time?
I plan to go with a data only plan and SIP service. That’ll also allow me to spoof my phone number to use the number everyone knows me by, without having to port it away from my Android phone. I don’t trust the Librem 5 to do LTE/VOLTE right now. But I hear that SIP is working well on the L5. The alternative appears to be going on to 3G and 2G for some or all of the time, if that still works in your area. I am concerned that the inferior coverage could result in lost or dropped calls, and lack of ability to make calls from some locations.
Have a look at keepgo and jmp. Keepgo uses the att network.
Mint Mobile is inexpensive and SMS/MMS works with the L5. I haven’t enabled LTE yet, so no personal experience there. However, there is good 2G coverage in my area for voice. 4G works great for data.
Of course… T-Mobile just bought Mint. No idea how that will go.
Probably any of them…
That’s interesting, I guess I need to look into what that is. 
I might be able to use L5 as my primary but whether or not I do, I still want a backup device.
What is this talk of 2G and 3G? I thought 3G was phased out last year, at least in the US:
OK, next items to consider are how much data you typically use per month, and whether or not you need multiple lines or just one. Also consider which features you need (i.e. international roaming, domestic roaming, rigorous account security/OTP authentication)
Verizon and any MVNO that works on Verizon’s network are non-starters with the current L5 modem, I think.
That leaves AT&T and T-mobile and the various MVNOs that use those networks. I know that the L5 modem’s IMEI passes muster with T-mobile’s IMEI checker, but last time I checked AT&T did not.
So you’re probably looking at an MVNO that works on T-mobile. BestMVNO is a good place to start for reviews of service and current deals. (Follow the T-mobile MVNOs link.)
I use Ting on T-mobile’s network. What I like:
.their customer service
.international roaming (paygo…OK for light usage; expensive for heavy usage)
.domestic roaming
.OTP authentication
.postpaid service
.cheapest plan is $10 for unlimited calls+texts, plus $5 per gigabyte (total for that is $15, plus about $1.40 taxes/fees)
Mint and Ultra mobile both have slightly better international roaming rates, but no OTP. (Note that Mint has had accounts compromised several times already.) To get their best deals you have to buy 3, 6, or 12 months in advance.
US Mobile and Red Pocket have some form of international roaming, but only by eSIM, which doesn’t work with the L5’s modem. Red Pocket offers year-long package deals on eBay. (I don’t like the fact that they have TikTok analytics software on their website!)
Tello has the cheapest build-your-own plan (i.e. x minutes + x texts + x megabytes), but not much in the way of extra features.
If you need free roaming in Mexico and possibly Canada, I think there are a couple that offer that. I don’t remember which.
Take your pick. 
Keepgo works on T-Mobile as well, turns out.
3G has been turned off most places, I almost never see it.
I don’t have VoLTE enabled – I’m waiting for support to get better. I get great 4G data. When a call comes in, it drops to 2G. Call quality and coverage are very good. And sometimes I even get 2.5G, as indicated by the notification bar.
(Edited LTE to VoLTE).
4G = LTE
LTE = 4G
EDIT: But I guess you were talking about “VoLTE.” Disregard.
There is also a 4th “major” carrier: U.S. Cellular.
They don’t sell service in every state, but their subscribers get to roam on the other major networks when traveling.
I’m not sure if their are any MVNOs that use U.S. Cellular’s network… I’m guessing not.
Yes, but I should have been more precise.