AsteriskBilling.com - General technical and billing information for VoIP operators using Asterisk PBX.
DID based prepaid billing works the same way as other forms of prepaid billing, but allows the user to purchase a DID so they can use their SIP devices to place outgoing calls and accept incoming calls as well. Their balance is calculated prior to placing the call and the call is dropped when the balance reaches zero. As with the other prepaid options, the user should be able to top-up their balance online at any time.
DID stands for Direct Inward Dialing Number (Also known as DDI)