What's the barrier for turning a 3G/4G Android Tablet into a Giant Phone?

Let's say that the 3G Nexus 7 rumor is true. What would be the technical barrier for turning it into a normal Android phone (no VoiP as a work around) if there are any? However impractical it may sound, I'm interested in knowing if installing the dialer, using a normal SIM card and activating with the Galaxy Nexus GSM Activation software would be possible. I know that it would be missing the older radio tech. For the sake of the question we're going to assume that the radio is a pentaband, HSPA+ radio.

I know that I'm missing something obvious here (no, not the mic issue, a bluetooth headset could resolve that).