said by TKJunkMail :

POTS isn't POTS either. If you are making a long distance call in the US, your call is being converted from analog into digital between cities anyway. The Earthlink svc isn't any different in that regard.

said by achuchma :

Do keep in mind when you use the POTS network in the US, voice data is usually transmitted on private, dedicated fiber trunks with no other traffic other than voice traffic. If mixed with data, voice will get the highest QoS.

VoIP runs on the "best effort" Internet, which is with a pot luck of other data...