There are significant regulatory issues involved. In principle it's possible (see osmocombb), but the legal hurdles around actually having user-controlled baseb

There are significant regulatory issues involved. In principle it's possible (see osmocombb), but the legal hurdles around actually having user-controlled basebands in a shipping product are significant. Of course, nothing says the baseband couldn't be open source, and even if codesigning is involved, the manufacturer could sign verifiable builds that can be reproducibly built. That should solve all regulatory concerns while still allowing users to inspect the baseband firmware for flaws or backdoors. But, of course, no actual baseband manufacturer cares about that.