GSM Prepay - credit left ? How does it work ???

[Tester2]

Hi,
I am a GSM prepay user. I activated the option to receive (hear) the remaining credit of my prepay account. What I would like to know is, how does this work ? Is my operator sending me this voice message based upon the credit they have somewhere in their systems, or is my credit stored on my SIM card, or is it a combination of the two... ?????
In brief... can someone explain me how that credit is kept, how the credit is checked, and how it gets to me on my phone ???
Thank you sooooo much for your replies....
Tester2

[cyclo]

The credit is stored on a server in your operators system. Each time you make a call, the server is checked to see if you have credits left or not, and the credit server is updated during the conversation.

[Tester2]

Thank you very much cyclo for this info. Do you also know if this information stays within the telecom servers, or is it also sent to the mobile phone owner ? Is there an exchange of this information ? Can a user capture it ?

[Kilrah]

Can be for certain operators, as for one here in Switzerland. When the phone logs to the network, it receives info about the credit left, which is then displayed on the phone's display. But it doesn't work for the 2 others operators.
If you could intercept it, you could only know how much money a person has, but this data doesn't travel in the other way (phone -> network). So you couldn't edit anything.

Regards
Kilrah

[Nanda]

Tester2,

prepaid credit information is stored in the operator's system, but some operators let users access their credit info through methods other than voice calls.

My operator for instance sends a SMS message after each call, WAP or GPRS access informing how much I had just spent and my remaining credit. It also lets me access my credit info though their operator menu (SATK SIM menu). Post-pay info is also available for contract customers (how much you have spent so far, internet access to the phone bill, etc.).

But all depends on the operator and what it wants to make available for its customers. There's no way you can pick that information up on the air, or hack your phone into doing it.

Hope this helps,

[cyclo]

As far as I know this information is never exchanged with the phone. The phone has no idea if it is a prepayed account or not.

[Nanda]

Prepaid credit credit information is not exchanged with the phone or SIMcard as if either was 'marked' or identified as a prepaid terminal.

With my network operator (Oi, 724.31) in the 'operator' menu (SATK application) there are both pre-paid credit and contract account status options on all SIMcards regardless of their 'prepay' or 'contract' status. In fact, you can change your number from prepaid to contract at any time, while keeping the same SIMcard since all this sort of data is both stored and precessed at operator system level.

Every time you place a call, use WAP or use GPRS on your prepaid phone, the operator server knows it's a prepaid number and generates an automatic SMS with information about the cost of last airtime use and credit satus.

And, by requesting credit status using SATK, you're sending a specific SMS request for credit information to Oi's servers, which in turn reply to your request also by SMS. That's the data exchange.

If you're in a contract phone and use operator menu option 'Prepaid credit', you get as reply a SMS message that tells you this information is only available to prepaid customers. Similar thing if you're on prepay and request 'contract' account status.

Phone has no idea if it's prepaid or not, but can exchange information with operator servers, which in turn DO know if it's a prepaid or contract phone and will send information accordingly.

Sorry for the looong post!

Cheers,