Connection problem or invalid MMI code

[#1]

I get the above pop up notification every time I try to use a USSD code from my daughter's Dell Streak, which is otherwise working well on giffgaff. When she sends a free text or calls a giffgaff number there is no "woohoo" pop-up message afterwards, or balance update. Her giffgaff sim has been active many months, and the goodybag is also on it's second renewal. USSD codes have never worked :-(

I've downloaded the giffgaffAPN app from the marketplace, run it so's to delete other APN's and tried single and twin APN settings, to no avail so far. (rebooted phone between each setting adjustment). Also I searched these forums for help! 

If anyone has any further suggestions that'd be fab!

[#2]

HAVE YOU ENABLED THE POP UP MESSAGES?

sorry for caps

[#3]

There are o2 upgrades going on in the following areas. Kent, Essex, London, Middlesex and Surrey. NO 2G or 3G USE in these areas at times

[#4]

https://giffgaff.com/settings

Try to turn them on from there !

[#5]

can you try the Sim in another phone,
see if it still happens,

[#6]

good suggestion! I tried the sim card in an old nokia, all USSD codes work perfectly. 

so I'll now check carefully through the settings in the phone menus. thanks for the help everyone, especially someone who suggested enabling pop-ups - if I can find the place to switch them on, I could be on a winner :-) !

[#7]

Try this :

http://www.appbrain.com/app/ussd-management/raj.com.ussd