[unobtrusive]

1. For the international SIM, say if I am in Germany and registered on T-Mobile with my UM SIM calling to Greece -- say to my wife on a UM card. What firm is "the originating operator"? T-MO, UM or the Greek firm receiving?

In this case the originating operator would be T-Mobile.

UM pays T-MO for the use of their network at rates set by T-MO. T-MO connects the call to your wifes UM number and pays UM a termination fee for doing so. UM then connects a call to the Greek network on which your wife is roaming and pays the relevant termination fee which is covered by the termination fee which T-MO pays to them. Your wifes free incoming is actually funded by your outgoing call charges.

On UM, the revenue to UM - prpbably shared with the actual service provider - is 25 ecents to setup and 39 ecents per minute. Now, that certainly will cover the termination fees in Greece and allow everyone a profit. But why should all the Greek operators allow UM free inbound?

As mentioned in another post, they aren't, it's just that the revenue that UM receives on incoming calls is enough to cover the cost of terminating on the Greek network. In the case that the origin of the call is also UM - then you have the following breakdown for a 5 minute call (forgetting VAT because that would just make it even more complicated);

You pay UM 2.20 EURO for the call (5 x 0.39 + 0.25)
UM pays T-Mobile 1.25 EURO for a call to another UM mobile (5 x 0.25)
A portion of that goes to UM for the termination fee, lets say 1 EURO (5 x 0.20)
UM then terminates a call on Vodafone Greece paying 0.60 (0.12 x 5)

So that's 2.20 - 1.25 + 1.00 - 0.60 = 1.35 EURO profit.

Obviously this is an example because I don't know what T-MO charges UM for using their network, also other networks have higher termination charges than those in Greece and we haven't taken VAT into account.

Consider if I used my DE T-Mobile SIM instead where I am guessing I would owe DE T-Mo upwards of 99 ecents per minute AND the Greek operator handling the call in Greece does not allow free inbound? If my wife was using our DE T-Mobile cards and received a call in Greece, my DE T-mobile balance is debited by 79 ecents -- whether I call from the UM card or a landline or pay the > 99 ecents from my other DE T-Mobile card. Where is the logic?
How is it possible that UM callers enjoy FREE inbound and DE T-MO users don't?

In part because DE T-MO demands a higher profit due to higher operating expenses (such as maintaining a radio network) and in part because they'll charge what they think they can get away with.

2. I don't understand how, if I am in France with my German T-MO card, the call "must pass through the users home network". I am registered on SFR, right? Someone in France using SFR dials me; how does that call pass through Germany?

That's just the way the networks operate. When an SFR user dials your German number, the call must pass through T-MO even if the call will eventually loop back to the same network.

Why? As I see it , the only way "passing thru" can happen is if SFR communicates behind the scenes with the German host network.

No, they just see that a call is made to a T-MO DE number and so passes the call to T-MO DE.

There is bound to be on the SIM card a set of approved roaming networks.

That kind of information is not stored on the SIM.

3. You do not address the possibility of negative balances; I THOUGHT I read this in DE T-MO literature about prepaid accounts, especially in connection with international roaming.

It would usually not be possible to fall into a negative balance because you have one of the following scenarios;

1. Inbound calls only - your home operator will disconnect any call that exceeds your credit

2. Outbound call using IN - the call will be disconnected when your credit reaches zero due to the real time billing

3. Outbound call using USSD - the call will be disconnected when your credit reaches zero because the call was originated by your operator, similar to callback.

4. Outbound call billed to a credit card - there is little risk to the operator, so you'll be able to make calls as if you are a contract roaming customer.

There is also the possibility of near realtime (warm) billing whereas there may be short periods where it is possible to fall into a negative balance due to the delay in update of billing records but I doubt many operators if any rely on this, due to the risk.