jsaxe did a great job explaining the 'DID's over POTS'. We have a couple of customers that can't get PRI but have the 'DID on POTS'. It works once it is set up correctly, but getting the setup working with the local CO was something I'd rather not repeat (and yes, they called on their OLD techs). If the PRI or T1 is an option, go that way. Good luck.