What is the 0091 (yellow) button for?

There are 2 buttons shown by default on your dialer: "050": the green button for IP calls (more economical rates) and "0091": the yellow button for voice calls through your mobile carrier lines (more stable quality). If you press the "0091" button, the 00912020 prefix will be prepended before your destination number, so that your call will go from your mobile line via Brastel.
The "0091" button cannot be used outside Japan or on mobile phones without voice plans. If you do not have a voice SIM, you may disable the "0091" button by going to Settings >> Preferences >> Controls >> Show GSM button >> OFF.

The dialing procedure is the same for both buttons. However, the calling rates differ depending on the area in Japan you call from. (See rates and area information.) Domestic calls placed with the "0091" button are completed through an international network and are therefore consumption-tax exempt. Rates for domestic calls (to or within Japan) placed with the "050" button already include consumption tax (except where noted).