DID is a way to directly call from a phone to reach a voip network.  The unique thing however, is that even though you might have a Michigan Voip Number reachable locally by normal Michigan phones, a DID number will allow someone, in Brazil for instance, to call a local number and still reach your Michigan number without paying any international fees.

To use a DID number you’ll need to know the correct number to dial which is not always the same as the number you dial on a traditional phone.

1)FreeDigits Free sips numbers and call numbers

2) IPKALL: a free did number for Washington state.
3)GrandCentral: Still in beta, but the ultimate one number for all phones.

4)IPComms:A free did with tons of choices.

5)Orbtalk: A free did for UK people.
6)Coms: Another good UK did number

7)Messagenet: Did numbers for Italy and UK folks.

Num2IM: A free France Did.

9) LocalPhone: LocalPhone offers a cheap international call service and also offers a UK Geographic number which you can forward to your phone.

10) Speex: A cool Israeli number.
11) TelFree: Did we say South Africa?  Awesome.