[OnePlus 6T  Questions & Answers] PanGalacticGargleBlaster:
Update, now on an AT&T SIM. This is a theme you should probably get used to.

Apparently the other day my phone was on HSPA when we tested, possible due to signal strength issues, which is why the calls came in no problem. I did a lot of troubleshooting by myself, with FirstNet support, and with a good amount of time spent by my local FirstNet specialist. Tried every setting I could think of including every preferred network type.

The issue in short...

On AT&T SIMs the phone starts on LTE/LTE for voice/data respectively. When a call comes in it switches to UMTS/HSPA in order to take the call over UMTS. The phone rings, and after the call it promptly returns to LTE.

On FirstNet SIMs the phone usually still switches to UMTS/HSPA when a call is incoming but the device won't actually ring. This behavior also occurred on another 8.0 android of mine. After the call it can take a protracted amount of time to switch back to LTE.

In my testing the number of calls that actually successfully switched from LTE to UMTS and then actually rang was about 2%.

More worrisome, the other person in my department who adopted FirstNet did so on an iPhone XS is FirstNet Ready. They still had issues and had to switch to an AT&T SIM, so it appears even officially supported devices are not a guarantee for full functionality. The Primary User Regional FirstNet rep was even indignant when I expressed concerns about the state of device support. FirstNet seems a bit of a half baked and not consumer choice friendly. I am also concerned about long term prospects given AT&T has something of a history only supporting certain features with first party versions of devices and not BYOD, such as VOLTE. Oh and to answer that question, no, VOLTE is not yet available on FirstNet for any device.

Ultimately is the AT&T SIMified FirstNet still worth considering? In my opinion yes. The pricing is on par with their prepaid plans, but unlike prepaid you're now put at the top of the priority list instead of the bottom and you won't be throttled. Yes, you still get prioritization with the AT&T SIM over other commercial traffic, though you won't ever trump FirstNet users and all your traffic will be handled by the commercial core, you don't touch the FirstNet core or band 14.

It is true you do not want to exceed 22GB for 3 months in a row. Our regional rep said you can get the "data-only devices" plan which he claims has no cap penalty, but my reading of the official terms and conditions seems to refute that.

Verizon is the other main option though last time we saw their pricing strategy it was more expensive and I don't recall any benefits over FirstNet other than possible coverage benefits. Then again slapping a Verizon SIM in your OnePlus will actually give you VOLTE2 without any of AT&T's shenanigans. Bonus Weirdness: When calling FirstNet's support line, if I called while my FirstNet SIM was in an iPhone it just told me they weren't open. If I called with my OnePlus it let me through. WTF.

So my take away is I'd really like FirstNet to make an earnest effort to make device support at least on par with the commercial network. I personally would not feel comfortable putting a FirstNet sim in anything other than a strict data usage device given the compatibility issues, and their device strategy gives me long term concerns. That said if AT&T has good coverage in your area, I'd at least suggest looking into the FirstNet plan with an AT&T SIM since it's still arguably a better deal than prepaid and consumer postpaid.

When it comes to agency paid plans though the story, at least from we're being told, changes a lot. Our Verizon rep says their non-consumer public safety lines are going to have no caps. I can find no such reference in FirstNet's terms. Our Battalion units on their current plans are throttled after 22GB and they still manage to use 50+GB. There's no way they could be put on FirstNet.