Don't quote me on this, but I know many CDMA networks don't like doing two things at once (voice and data, voice and text, etc.). Although, this is just an alert 'noise' if you are getting the text anyways that statement is irrelevant but that
is the only difference that could happen between the two carriers. Now, I did some Googling and found this app:
SoundAbout - Android Apps on Google Play Supposedly, it allows you to route the phone at your leisure to wherever you want. I haven't played with it myself but give it a shot and report back because...I don't know if there's an internal system setting to change that.