AFAIK, no ILEC for one given area acts as a CLEC in another. If you are in a Frontier region, who knows what billing mischief you may create if you attempt to order CentutyLink.
You could look for a CLEC which provides service akin to Sonic.net "Fusion" service (California only), or G4 Communications (New Hampshire).
If you have
DSL Extreme as a CLEC option, they likely just resell Frontier service.