Anything wrong with using an unlocked Docomo phone with At&t?

Question:

Since Docomo phones sold after April are sim-unlocked, If I were to purchase one that supports At&t bands from a Japanese online retailer, have it shipped to the US, and then replace the sim card with the one I'm currently using, would I theoretically have a fully working Docomo phone able to make calls and send texts with no "international" fee?

Answer:

Your phone will be on an AT&T plan when you put the SIM card in it. Whatever fees charged will be according to your AT&T contract.

The DOCOMO phone has nothing to do with the fees you get charged, your AT&T contract does.

Essentially it is the same as if you purchased an unlocked SPRINT phone and put your AT&T sim card in it, your AT&T charges still apply, it doesn't matter if it was a sprint phone.