As I recently learned, there are a number of people in the HAM radio community interested in operating their own TETRA networks. Some of them actually interface commercial TETRA BTS via their proprietary E1/TDM based back-haul interfaces, pretty much how we started with OpenBSC back in 2008 implementing the GSM Abis interface of Siemens GSM BTS.
I don’t know much about those developments, but they are aparently using the Osmocom icE1usb, as can be seen in the tetrapack.online wiki at E1/T1 Interface | Tetra
Some of the software appears to be available from Index of /repository though I somehow had the feeling it’s sadly not FOSS software (I may be wrong!).