Taking out the SIM card is not really a solution for most of us, and really shouldn't be necessary.
Curiously, when you use either of the functions intended to give you ID information about the Telemetrics system, the SIM number it gives you is one digit shy of a true AT&T SIM number. Those are 20 digits, and the Leaf only yields 19. I wonder if this is intentional or if this is the convention in Japan (19 digits). In all cases, the SIM can be translated into the phone number by personnel with access to the AT&T database, but without the entire SIM, there is no way to accomplish this. I had an AT&T tech try mine by adding each of 0-9 to the end of the 19-digit SIM number with no luck.
If you use ID Check, the number the unit gives you as the Unit Telephone Number is about 15 (can't remember exactly) digits--more than a US standard 10-digit number, but also more than an international number. Additionally, the first four digits are 8823, which cannot be made into a country code. I believe this number is actually the MSISDN, which I do not think is the same as a phone number, and about which I know much less than about SIM numbers.
It's a puzzler that I wish I could solve.