It is SMS for the bearer.
They have configured their OZ server to use SMS. It usually does it over GPRS and then falls back to SMS. This allows you to get IM's while on the phone, or when you drop out of a GPRS service area but still have GSM service, etc.
Which is nice and all, but really it's a revenue grab by T-Mobile USA.
So either buy the unlimited package, or pay those nickles. I'll see how it goes on my first bill, had a huge issue this morning that will require some attention.
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