Phone tip: selective silence mode

I thought of this myself, and I haven't seen it anywhere else, so it should be pretty new.

What it does:
Like silence mode, your phone will not ring when calls come in, except for calls from selected numbers.

• You want to sleep, or otherwise not be disturbed, yet you don't want to miss important calls from family.
• You asked for a wake-up call, but are afraid of being woken up by other calls in the middle of the night.

Requirements:
Phone that allows you to set custom ringtones for different people.

How:
You need a "blank" sound file. The easiest way is to use the voice recorder on your phone in as quiet an environment as possible. For a totally blank file, I used a recording program (Sound Recorder on Windows) and set the input source to line-in (which I left unplugged).
Set your ringtone to this blank file - when calls come in, the file will play, which should not produce any sound.
Set custom ringtones for the people you want to receive calls from: family members etc. When these people call, their ringtones will play instead of the blank file.