I decide use GSM channel for local Asterisk PBX. Many info about connect GSM phone via BT or Chan_sebi/gsm_open and other. Only one problem - i havn't BT and modem.
So i decide use devices which i have.
First - Maple dev board (STM32 based) and UBlox proto board (LEON-G200).



Voice channel use I2S link and represents as ALSA under Linux.
UART link used for control/status.
After some modifications (and additions) in chan_sebi i can use this device as FXS/FXO in Asterisk.

Current version realize on two boards - Maple and UBlox (2 and 3 in post) but later i think make a USB module in a ~ 25x40 mm