[unobtrusive]

I used to work in the prepaid billing dept of a mobile operator, so I think I can add a bit more detail to this discussion.

Firstly, all mobile operators (and fixed operators even) charge a termination fee to the originating operator of an inbound call. In Greece for example, that fee is currently 0.12 EUR for Cosmote and Vodafone, 0.125 EUR for TIM and 0.17 EUR for Q Telecom. These fees are a major source of revenue and are charged no matter where the call originates, except possibly in the case of large operators like Vodafone. A call from Vodafone UK to Vodafone Greece for example, may either avoid a termination charge, or more likely, the termination charge is made but the money stays within the Vodafone group anyway.

Now taking the example of an International SIM that allows free inbound to Greece - as long as the revenue to the originating operator for the incoming call to its own network is at least 0.125 EUR then free inbound can be offered to the 3 major networks without suffering any financial loss. If the inbound revenue is higher than 0.125, or if the destination networks termination fee is lower than 0.125 then a profit is made.

I think this goes a long way to explaining the very large price increases a lot of operators now charge on calls to Leichenstein mobile numbers.

On the subject of how prepaid international roaming can be offered and whether an operator can know the balance of another operators customer, this is a bit more complicated.

First I'll take the case of an incoming roaming call. This can be offered simply because the call must pass through the users home network and the home network stores the balance of the users account. When that balance reaches 0 the call can be terminated and users profile changed to disallow further calls.

A lot of operators will allow incoming calls on prepaid but either not allow outgoing calls at all, only allow calls from some operators with which they have SS7 links, only allow outgoing calls when USSD codes are used and the home operator originates the call (and so can monitor the customers balance) or in some cases only allow outgoing roaming calls when they are seperately billed to a credit card, removing the need for realtime billing.

In the case of an outgoing call where the 2 operators involved have an IN/SS7 link, IN services are used to allocate time slices to the roaming user. For example, when the user places a call, a request will be sent to their home network to verify that the user has sufficient credit to initiate the call, which usually with a 30 second or 1 minute minimum duration. After the initial duration has been reached, a further request will be sent to verify the user has enough credit for a further x seconds.

This process is repeated throughout the call until either the call is terminated or the user runs out of credit, with the home operators billing system charging for the time slices as appropriate. There is no sharing of customers balance between operators.

In some cases it is not possible for operators to provide roaming this way due to the combination of network and system latency. Usually a request must be answered within 100ms and the combination of network latency (e.g Internet) as well as delays caused by system load (CPU, database, disks) can exceed this time.