If you cause the device you own to operate according to your own will (i.e. the core concept of FOSS) instead of the will of the carrier, there is a strong likelihood it will cause a denial or degradation of service for everyone else.

Verizon has the right to transmit on spectrum allocated to it using consumer devices as its agents. It employs engineers and QA processes to make sure that any device transmitting on its spectrum plays well with others before it is allowed to leave RF-isolated testing facilities.

The public does not and should not have the right to transmit on Verizon's spectrum, even using devices they own which are legally and technically capable, except according to Verizon's carefully vetted programming. If they were able to run their own radio firmware, you'd have the situation described in the parent.

Cellular radios necessarily cannot be open source. The source could be released for inspection and audit, but it cannot be possible or permissible for you to run modified source on "your" radios.