Call forwarding with GSM codes

Hi, I am trying to use GSM codes to set an iPhone 4 to forward calls when unanswered in four seconds, and when busy.

I have tested the codes on another iPhone and it has worked fine but it will not work on this phone!

It is a French iPhone 4 on the network SFR. The codes I am using are:
*61*[alt.number]*4#
and
*67*[alt.number]#

If anyone can help with this I would be very grateful!

Thanks.