There are two types of messages updating you on your current credit balance:
1) messages after each call
2) messages after each SMS/MMS message and/or data connection
1) in order to de-activate messages after each call, dial: *106*10# and then press the call key (if you wish to re-activate those messages, dial *106*11# and then the call key).
2) for de-activating messages after each SMS/MMS/data connection (in case this option is activated), dial: *106*20# [call key] (to re-activate: *106*21# [call key])