Yup that's what you want, as an added measure I went ahead and registered my phone number on the
https://mymedianet.wireless.att.com/ website while my SIM was in the other phone.
Before when I had the iPhone picture on my AT&T account it would give me an error when I would go to the medianet website. However once I manually switched my phone on the website and send out a couple of txts and made a couple of calls with the other phone I was able to enter my number and received a txt msg code that I typed into the media net website to activate media net on my account.
Like I said in the post that I linked, I did both things (registered phone # with Media Net & switched to a non iPhone messaging plan) and I was able to send receive MMS.
I did not receive a any txt message saying that MMS was enabled on my account, it just started working.