Garmin GTU 10

Hi everyone.
I was wondering if anyone could help me out with my understanding. I have GTU 10 and it looks like Garmin will stop providing service for this device by January 1, 2016. If i understand it right GTU 10 has sim card from ATT. Now, if this is how it works then what if i can change their sim for my sim and use it not through their app, but through find my prone? will it work?

Thank you.

Is ATT the same as AT&T ?
At Adafruit they used AT&T sim cards for modules, but now I read this red warning : Obtaining a SIM | Adafruit FONA | Adafruit Learning System
It seems that AT&T drops the 2G GSM all together. Bad luck.

As far as I know a sim card is a sim card, and can be used for GSM phones as well as for other devices. The sim card is just the subscription for at least GSM and SMS (and maybe more services like internet and so on). But I'm not 100% sure of this.