I ran into an issue today, when I attempted to start browsing at some stage today. I was diverted to a web page to alert me I had run out of data. Nothing would cajole the phone into accessing anywhere.
However when I checked using the ussd menu, I confirmed that (as I suspected) I still had plenty of data, but my credit balance was only 10 cents.
I then added $10 credit and my data started working again.
Is this a coincidence, or is there a minimum credit requirement to start a data session?