DID stands for Direct Inward Dialing, while DDI means Direct Dial In, and both acronyms refer to the same thing: A phone number that will lead incoming calls to a device. In our case, a call to that number will be ringing a SIP device.
Normally a customer will port his/her pre-existing PSTN number(s) to his/her ITSP (that is, the customer's number will not make the landline ring anymore, but will ring the end customer's SIP device passing through the ITSP SIP network). ITSPs often have a specific branch of their customer service assisting in the number porting procedures.
DIDs are sought by customers for many reasons: As a primary way to get incoming phone calls (for example, the main phone number of a person or a company, if they have no previous number, or don't want to port it), or as a means to be present in local, regional, or international markets, so as to allow the public to reach a company for the cost of a local call, or to be compliant with regulations...
