I remember in the mid 2000s talking with a ham operator. The bands didn't overlap, but they were interleaved - the radios could access bands reserved for cell calls. If you snipped the right resistor off the right radio, then you could listen in on cellphone conversations.

Newer iPhones support band 42 TDD, which is 3400 to 3600 MHz. The amateur radio 9 cm band in the US is 3300 to 3500 MHz, so you could have a setup contained in single ham band. However, the open source eNodeB products typically don't support TDD.

This is the CBRS band, and the idea behind CBRS is that people should be able to gain access to it on an ad-hoc basis by making a request for it, when it isn't being used for marine radar on the US coast.

I believe OpenAirInterface can handle TDD, although it was firmly "research grade" code last time I looked at it.

The positive from CBRS is that it should (or at least is intended to) spawn a new generation of lower cost small cell base stations, using this band, and speaking the CBRS "protocol" for spectrum access coordination. And that has potential to help reduce prices of radio equipment.

Handset compatibility is coming on this band quicker because some existing mobile operators have purchased PALs (priority access licenses) for CBRS spectrum, and intend to use this for some extra capacity.

P.S. just as a very minor technical correction, CBRS is defined for band 48, rather than 42, although with some overlap. B48 is 3550 to 3700 MHz, while B42 is 3.4 to 3.6 GHz as you said. Therefore when looking for devices, it's best to look for B48 (although at a push, if you're doing your own R&D, B42 will be fine for use in the lower 50 MHz section of the band).