Perhaps the modem certification might be enough. But I am not sure about that. Actually, anything that transmits any kind of radio frequency in the US has to receive a certification from the FCC to be legal. This should include both the modem chip and also the phone itself. Each different radio service has different requirements per applicable statutes that are specific to that radio service. The only radio transmitters that I am aware of that do not require ‘type acceptance’ of the radio in the US are amateur radio transmitters.
The ‘type acceptance’ is achieved by having an independent certified laboratory run required tests and then signing-off that the given radio passed all of those tests. Then the FCC will certify the radio. The tests are both technical and functional. For example, a citizens band radio can comply with all technical specifications for RF emissions and still fail if (for example) the CB radio is capable of being used for a different radio service also, without internal modifications. For example, it is easy to buy a non-certified radio from China (right on Amazon even) that will transmit and receive on many different radio service bands all from the same radio (commercial, amateur, GMRS, MURS, marine, military, police/fire, others). But because it will work on more than one radio service, it can’t be certified for legal use in any radio service. This requirement is for political and not technical reasons because there are no technical reasons to not allow this. Likewise for political reasons, cell phone radios might have politically driven statutory requirements to receive an FCC certification. Sometimes it may not make sense to the average person why some requirement may exist.
I have had some concern about the following. If the modem in the Librem 5 is capable of transmitting outside of legal cell phone frequencies, there could be legal road blocks to getting it FCC certified. In VHF and UHF transceivers, all transmitter and receiver circuits are made to cover the whole (or most of the) VHF or UHF band. The radio can only be certified in most cases, because the software or firmware prohibits out-of-band transmissions. So if a taxi company uses UHF radios to communicate with their fleet, the proprietary (closed source) software or firmware prevents the radio from transmitting on police or fire department frequencies. Under these conditions, the FCC will certify the radio that is sold to the taxi company. If the company that sold the radio to the taxi company were to use open source software, would the FCC still certify that radio? Pretty much any programmer can alter the taxi company radio in that case, to transmit on police frequencies. With the police and fire companies migrating to higher frequencies and trunked systems now, the FCC is loostening-up on enforcement lately. You can even buy a Chinese UHF radio now that is certified by the FCC (under Part 90) for commercial use and that also has a VFO (a big no-no for anything except amateur radios) that can be unlocked. So there are now commercial radios that can be unlocked to dial-in any frequency from a key pad instead of having locked-in channels. That wouldn’t have been allowed thirty years ago when police and fire departments relied on their use of VHF and UHF two-way radio systems.
If a modem in the Librem 5 could be programmed to operate outside of legal cell phone frequencies, there could be a problem. With open-sourced software or firmware, anyone might be able to program the modem to operate lillegally. There are no lock-outs to prevent this. In addition, the cell phone companies might have valid legal grounds to ban the use of these phones on their system if the software or firmware could tune the phone to do things that compromise their cellular phone system. For technical reasons, all radio circuits have to be made to operate outside of the band of their intended use. Like a car engine, if you want it to work well for its intended use, it has to be capable of greatly exceeding that intended use. In this metaphor of the car engine, only a governing device could render the device incapable of illegal use. Such a governing device in a radio transmitter would have to be proprietary locked firmware. To design a radio to fail just outside of its legal operating frequency would be like designing a car engine that runs well up to the speed limit, but that fails for technical reasons at only one mile per hour above the speed limit. No, you need governing firmware to do this in a radio transmitter. Whether or not the FCC will allow that firmware to exist in the public domain in a certified radio is the question. The only other alternative would be to trust the modem manufacturer to have proprietary locked firmware that we have no access to.