Billion Electrics manufactures various ISDN USB modems
from most advanced BIPAC-221, by BIPAC-211 and mini USB
to tiny USB. These modems are selled also under Trust
and Microcom brand. They work on Siemens HFC chipset
for ISDN functions and Intel for USB. They understand
AT commands, and they semi-active (probably voice
Because i have not those type of programmic skill
please write drivers for this type of hardware.