how to select network mode

*#*#4636#*#*

In the "phone Information" menu, you can select the following:
(note that this is my own understanding, I might be wrong)

GSM Only
2G only, the phone will never be on 3G, even though there is no 2G available, unless it can find 2G on a roaming partner of your operator.
Risk: the phone will go into no service is no 2G available, or if no 2G is available with the roaming partner

GSM Auto (PRL)
The phone will stay in 2G as long as possible. When there is no 2G available anymore, it will go to 3G on your home network, and if not possible, it will try to use the 2G network of a roaming partner (another operator). Last solution will be to use 3G on another roaming partner.

WCDMA Only
3G only, the phone will never be on 2G, even though there is no 3G available, unless it can find 2G on a roaming partner of your operator
Risk: the phone will go into no service is no 3G available, or if no 3G is available with the roaming partner

WCDMA Preferred
The phone will stay in 3G as long as possible. When there is no 3G available anymore, it will go to 2G on your home network, and if not possible, it will try to use the 3G network of a roaming partner (another operator). Last solution will be to use 2G on another roaming partner.