Lots of different ones. Here in upstate NY it's Paetec but they have lots of different deals with different CLECs. If you're wondering and have a lot of spare time, visit their voicemail help page atwhich has a list of their voicemail access numbers which are in the same number blocks as their phone numbers. Look up the NPA-NXX of the voicemail number to see what CLEC they use in that area.
John R. Levine, IECC, POB 727, Trumansburg NY 14886 +1 607 330 5711 email@example.com, Mayor,Member, Provisional board, Coalition Against Unsolicited Commercial E-mail