gsm850 and host/layer23/src/mobile

I'm having issues getting the mobile application to work and I'm wondering if this is a gsm850 problem. I have a couple of debugging ideas but I thought I would post here just in case the answer is obvious to someone else. I'm testing on both a c118 and a c139. I'm using the remotes/origin/sylvain/testing branch and this does have quadband support. I've edited target/firmware/Makefile and defined CONFIG_TX_ENABLE. I have a valid SIM in my phone that works with both phones using the default Motorola firmware. The 'sim reader 1' command is able to gather (most of) the necessary information. (The access class is not correctly read from the SIM but I haven't bothered tracking down why yet -- I just hardcoded an access class of 0xffff.) The mobile program correctly scans the frequency ranges and finds a good tower and then attempts to do a location update. It generates a RACH channel request and then (appears to) send it. The problem is that the phone never receives an immediate assignment response. The log shows multiple immediate assignments received, but none match the request reference (RA and FN the burst was sent in.) So it appears that the tower never receives the channel request I send. The only test I have done so far is to monitor the uplink frequency with a USRP (uhd_fft.py) during the location update procedure. It looks like the phone is actually transmitting at the correct times, but I am not positive. (I'd have to write some custom code before I can definitively say one way or the other.) The only other idea I have right now is that perhaps the timing advance calculation is incorrect. There are a couple of constants used without any explanation and I guess it is possible that these constants don't work for my configuration. I'm going to start going through these constants to determine if I can find a value that works. Any other ideas to try here? Anyone else gotten this to work in the US?