I can tell you how the telecom world works, as I have many years working for CLEC's and dealing with portings of both residential and businesses ranging in size from 1 line to 1000+ lines. Most companies use shared circuits, meaning your phone line and DSL run on the same copper loop to your house. If your DSL was not stand alone and was a shared circuit than your DSL number WAS your home number as well, and porting out the home number technically would wipe out the DSL circuit as well (means someone over looked the port out). If you have a stand alone DSL your DSL line has a number unto itself (you usually see the number next to the DSL billable on the bill).

VoIPo isnt at fault AT&T failed to check what was going on with your services before porting out. What you need is called stand alone DSL circuit. AT&T will know what that means, and if you mention there oversight this should get fixed fairly fast if it hasnt already.