[andy]

How is it financed, or how does the SIM work, or both?

Obviously there is some incoming roaming cost, but these operators have taken a view that for marketing reasons they can swallow this in return for the income from the outgoing calls, or that the received incoming termination fees also subsidise it to some extent.

As for the technical detail, they all use callback; when a call is set up by dialling a number as normal, the user sees a screen message to please wait. Then the phone maybe actually sends something like *1xx*00nn...nnn#, where the 1xx varies slightly on different providers, e.g 117 or 137, and the 00nnn after the second * is the desired destination number. After a few seconds this user's phone rings ...

Some phones, or one or two of the SIMs, don't support dialling the destination as an ordinary number like that, but the code can be sent as above

Or other systems may be actually sending an sms format to trigger the call, but either way it's usually invisible to the user

With some of these, the user's phone number doesn't actually pertain to the country of the underlying SIM card, with either a landline or mobile DID forwarded to it.

Someone else can probably add a bit more technical background, or at least the right jargon sets of initials