Currently on iOS 8 you cannot request USSD for your balance with the iPhone which is unlocked by using GEVEY Sim or GPP, Luckily there is a workaround for that :
1. Add this repo in your Cydia http://bovor.net/ios
2. Search for this program Fix USSD iOS 8.x (ទ្រនាប់សីុម)
3. Click Install and Confirm
4. Reboot your device and try to request the USSD for your balance
If the above method is not working for you try this following instead :
This guide will help you to use USSD code for iPhone unlocked by sim interposer .
1. Goto Cydia to install iFile .
2. add this repo http://cydia.myrepospace.com/applelab
3. install CommCenter Patch iOS8 . ( if you got "size mismatch error" , use this repo instead "http://DJHartley.myrepospace.com/" ) .
4. Open iFile --> browse to /System/Library/Carrier Bundles/iPhone/Default.bundle/
5. Click on carrier.plst and select "Viewer PList" .
6. Turn ON "SupportsSupplementaryServices" , click Done .
7. Reboot your iPhone .