I have a Pinephone Pro on Verizon in the US. I know it’s not the same as the Librem 5 but I’m still waiting on mine to get to me. I have a difficult time using the PPP on Verizon’s network but it mostly works. I get text messages (not MMS, but I think there’s a different issue than the network), I can make and receive phone calls on VoLTE, and I can browse the web on the cellular network.
I did as @StevenR suggested and activated my SIM card in an “approved” phone; but almost instantly after I moved the SIM to my PPP I got an email from VZ that said they noticed my SIM was moved. In addition, my VZ account online shows an unrecognized device as my line and has a curious display that says the phone calls are deactivated and that it’s a data-only device. I assume that means only 4G VoLTE will work but I knew that anyway since VZ’s 3G network is CDMA and the PPP has a UMTS modem for 3G.
The point is, moving your SIM from an “approved” phone to a “non-whitelisted” phone will be noticed by Verizon. I don’t know if you can activate a brand new SIM in a non-whitelisted phone or not but I don’t think it makes much difference if you use an approved phone first; they won’t be fooled.
I’m pretty sure the modem registers its IMEI with the network and that’s out of the user’s control since all modems run at least some closed source software. I wouldn’t be surprised if the network negotiation and device identification bits were part of the secret sauce but I can’t say for sure. The community expert to ask would probably be Biktorgj.