If you receive a voice call and the machine is in F/T mode, it will start to sound the F/T (double) ring after the initial ring delay. If you pick up the call on an extension telephone you can turn the F/T ring off by pressing # 5 1 (make sure you press this between the rings).
If the machine answers a voice call and pseudo/double-rings for you to take over, you can take the call at the external telephone by pressing Tel/R.