that OsmocomBB project might not be designed optimally
for US GSM Bands (GSM
850/ PCS 1900). The system
could not get synchronised to a frequency when FSBS request is carried out.
Well you need to enable it in the configuration for it to even try those bands.
In the telnet vty interface try something like
enable
conf t
ms 1
support
gsm-850
pcs
write
Cheers,
Sylvain