As promised:
UPDATE 29/03/2013: SMS delivery issues resolved with a workaround.
https://github.com/fairwaves/openbts-2.8/commit/c617e53a2c2eccfdf458021a981fbe94b9d488dc
We were recently looking into a bug when MT-SMS delivery was interrupted with an unexpected UA frame on L2 layer. We’ve fixed this in our branch by ignoring a second UA in Established L2 state [1], but the real issue lies in the filler table coupled with delays in processing in higher layers.