i leave that question to NSS gurus…
but let’s focus on the BSS part:
in the drivetest, you should see (in case of originated call)
sdcch:ciphering mode cmd/cmp
sdcch:call proceeding (dl)
sdcch:assignment cmd (dl)
tch:assignment complete (ul)
there is no channel assignment on the air interface. the alerting will come after the MS has sent the assignment complete.