Tokyo_Dom is correct. Softbank do not unlock any phones.
Because when I called I told them exactly what was my problem: "Right now I'm living in France and with my phone I can't use my operator's SIM card. Could it be possible to arrange this?"
And they answered me: "Yes, we can. Please tell us your phone number (a Softbank one I don't have)."
Since I don't have any Softbank SIM card, I talked with the one who answered me and told me "Yes, if you had a Softbank SIM in your phone we can help you."
So, if I had a Softbank USIM and told them my phone number, they were okay with SIM unlocking it so that I could use it with a French operator. We were not talking about using it without a SIM card since that's what I'm doing everyday...
They were talking about enabling network roaming. Thats all. Nothing to do with the phone.
Thats why they could do it over the phone. Thats why they needed your phone number; simply to pull up your account, enable international roaming and roaming data.
On most worldwide network locked phones, when you put a different SIM in the phone you get a message saying locked and then it asks you for a network pin. This pin can be bought from the network operator which supplied the phone, or in some cases you can find some place that will do it cheaply.
On all Japanese phones, putting a different SIM in just results in a message saying "check USIM", no option to put in a network code, no ability to do anything other than turn off the phone and try again with a proper SIM.
So the unlocking process will need to be something that is done physically; through some kind of USB hack or something