Virgin Mobile runs off of the Sprint network and AT&T runs off of their own network. Sprint uses CDMA technology while AT&T runs off of GSM technology.
These two technologies are normally not compatible with one another as they are designed with unique band frequencies.
Virgin Mobile phones are Sprint-branded phones. That means in order to use it on another carrier, that carrier would need to be compatible with the Sprint network. In other words, Sprint branded phones can only be activated with the Sprint network or other Sprint MVNOs.
Take a full look at all the latest Sprint plans and Sprint MNVO plans today.