If all trunk - OS (out of services).
Sometimes - in trunk you have declared "trunk COS"
External services/ Trunk COS - for used COS manage item "Unavailable Time Betw.2 Seizes" - try with 20 (or 25) instead of 10 (1 sec). You need to reset board after this.
Was problem with NDDI board - 1 sec not enough for release and second seizure.
If all trunk - busy - check why (f.e. with listincall command).
Transfer with two external user allowed (DTO joining in TG local parameters)? If yes - system/ other system - what about "No detect.of On-hook tone" item. And parameters for - System/ tone detection parameters - parameters for "on-hook tone".