You're not going to get open firmware. Cell phones are radio transceivers in the hands of millions of unlicensed users. The only reason any of the cellular infrastructure works is all the devices run tested and certified firmware.

For most cellular basebands the PHY is more powerful and capable than the regulations would normally allow. They only get certified for consumer use because the firmware bounds the operation to the regulated envelopes.

No devices would ever be allowed to be sold on the consumer market if end users could go tweak the firmware or run uncertified firmwares. They shouldn't be allowed to be sold. The difference between a transmitter and jammer is a pretty fine line.