There have been reports of problem with RFX 1800 boards running OpenBTS.
Due to the large clock drift of the USRP crystal, the frequency error of
the transmitted signal can be larger than the capture window of the
handset, and thus the handset can not find the basestation.
There are a couple of independent options for dealing with this:
1) Rework the USRP to use an external clock reference that is more
stable. Instructions for this can be found in the USRP/gnuradio wiki
and associated documents or mailing lists.
2) In Transceiver.cpp of the Transceiver directory, there is
#define FREQOFFSET xxx
where xxx is a frequency shift in Hz. Adjust this until the phone can
capture the signal. Usually the phone will only search +/- 10kHz from
the nominal center frequency of the ARFCN.
This problem is far less prevalent with the RFX 900 boards, since the
frequency error will only be one-half that of the RFX 1800 frequency