DNIS (or Dialled Number Identification Service) is a feature of the telephony network that enables us to know which number was dialled by a customer so the appropriate Call Routing strategy can be applied.
How does DNIS work?
To give you an example, your company might have several 1800 numbers for customers to dial, perhaps one for each geographical state that all come into the one call centre.
By using DNIS you can determine which 1800 number they called and use this information when routing your calls to the agents.
This can be useful if you have experts the specialise in each state for example.
You can also use the knowledge to play different hold messages, queue with a different priority etc.
Pretty clever stuff.