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 2 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 will send the call to your PBX over the SIP Trunk. Your PBX can then route the call to an extension based on the number dialed.
List the SIP configuration guides section of the docs to get 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 will send it directly to your SIP registered endpoint.
See documentation for registering a SIP endpoint on Twilio.