In my yallo under "Settings", you will find various cost checking options. 

090x numbers and Premium SMS:

When this option is deactivated, premium rate numbers (hotlines or teleshops, etc.) can no longer be used.

Mobile Internet:
If you deactivate mobile data, you can only surf the Internet over WLAN.

Use abroad:
After deactivation, the use of mobile data, SMS/MMS, and calls will be blocked.

Data roaming in excess of options and included data:
By deactivating, you will avoid paying for mobile data on the Internet once you have used up the options and included data you have signed up for. You can continue to use this as before.

Tip: To save costs, take advantage of our attractive add-on packages.