I have recently changed my GSM provider to a new one. Everytime i make a call i get this annoying message, but theres a fix to it.
There’s an easy way to remove this is by using GSM codes:

After you press call, you should get this message:

Press dismiss and try making your call, the message wont appear.
This works with 2.x and 3.x , which were tested.
Doesn’t work on 3G with 3.1.2
Fantastic! Worked perfectly! Thank you very much!
not work.
It didnt work for me either
I’m have a 3Gs with 3.1.2
Not working for 3GS 3.1.2
Wicked!! Work Perfectly Fine With 3.1.2
Worked fine with me, thanks man!
Worked a treat on Jailbroken 3G on Vodafone UK… Cheers
Thanks, worked perfectly for me on Vodafone with jailbroken 3.1.2
thank you soo much!!! worked on my iphone 3gs 3.1.2!!!!
dosn’t work on 3.1.2 GS
Worked perfectly on my 3G with 3.0.1
Does not work on a 2G with FW 3.1.2