With all the problems O2 are currently having, there is a way you can divert your calls to another number. I had to do this as I am on call for work and had no signal yesterday evening/night.
There are a few ways to do this, the mail one you may want currently is this one below.
Divert all calls
To activate this ** 21 * phone number # SEND
To Cancel this ## 21 # SEND
To Check the current status * # 21 # SEND
The other options are
Not Reachable Divert
To activate this ** 62 * Phone number # SEND
To cancel ## 62 # SEND
To check status * # 62 # SEND
Divert Calls when busy
To activate ** 67 * 901 # SEND
To cancel ## 67 # SEND
To check status * # 67 # SEND
Divert Calls when there is no reply
To activate ** 61 * phone number * 11 * # SEND
To cancel ## 61 # SEND
To check status * # 61 # SEND
I am always interested in your thoughts so if you have any comments or feedback then please feel free to add any comments, or you can mail me here.