in my mind, “no service mode” mean :
1/ the DL RXLEV goes below the RXLEV ACCESS MIN defined in the cell (or more precisely : the C1 becomes negative)
but maybe (and I strongly insist on the “MAYBE”) it could also mean :
2/ MS cannot decode the BCCH system information messages (due to strong DL interference on BCCH frequency)
3/ the cell is barred
(to be checked with MS’s software expert)