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