After reading this, I just started experimentally sending all calls to voicemail. Here's how I set it up; it should work with GSM phone carriers:

- Dial the below number. It'll show a bunch of information, including your voicemail number. Remember/copy the number.

    *#67#

- Create a new contact. I put "Phone Off" in the company name field. Use the below number as its phone number, replacing the "X"s with the number from the first step, including country code. Calling this contact will tell the network to forward all calls to voicemail without the phone ringing.

    *21*+XXXXXXXXXXX#

- Create another new contact. I put "Phone On" in the company name field. Enter the phone number below. Calling it will cancel call forwarding.

    ##21#

I added both of these contacts to my favorites and can tap them to enable/disable calls. Right now, I'm only planning to turn off forwarding when I'm actively expecting a call. It turns out that my phone puts an indicator in the status bar when forwarding is turned on, which is cool. Let's see how it goes.