iPhone Activation help

So I bought a 5C and it arrived today, but I threw out the sim card that it came with thinking I could just pop in one that I got previously from the ATT store in it to do a cross upgrade. Now I am getting the error that it cannot activate and it is saying "The SIM in your iPhone is not the one that shipped with it. Please reinsert that SIM and restart the iPhone." I have no way of retrieving the SIM card and I went to the ATT and Apple store and they could not help me for 3 hours.

Should I just exchange the 5C for the same 5C and restart the process?

Applejuiced said:

How did you get the phone? Upgrade or bought it brand new full price?
I don't know why they won't help you if you just lost the sim.

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Upgrade on a different line, took out the sim from the phone, and put in one that I got from ATT. iPhone won't activate since it will only activate with the SIM that it originally shipped with but that's long gone in the trash.