What is Direct Inward Dialing (DID)?
Direct Inward Dialing (DID) is a telephone service that allows a phone number to ring through directly to a specific phone at a business instead of going to a menu or a queue and needing to dial an extension. A phone number that is used like this is often called a "DID" (and multiple numbers are called "DIDs").
There are two ways to get DID-like functionality with Twilio Phone Numbers: via a SIP trunk, and with SIP registration.
You can connect a Twilio SIP Trunk to your PBX. When a call comes in to Twilio to any of your phone numbers, Twilio sends the call to your PBX over the SIP Trunk. Your PBX can then route the call to an extension based on the number dialed.
See the SIP configuration guides for step-by-step instructions.
Using Twilio SIP Registration, you can connect a call directly to a VoIP desk phone or soft phone. When a call comes in to Twilio to your phone number, Twilio sends it directly to your SIP registered endpoint.
See documentation for registering a SIP endpoint on Twilio.