Different VoIP service providers have different ways to associate phone numbers with the URL of specific sites. In Twilio, for instance, one is expected to fill in a special form indicating which URLs to use for "incoming calls", "incoming text messages" and "call hangups". Please check the README.txt files associated with your VoIP Drupal server module for additional details.
On the Drupal side, one has to tell the system which script to activate whenever somebody calls the website, i.e., whenever the URL associated with incoming call is accessed. That is done by going to Administer->VoIP Drupal->Default call configuration (admin/voip/call/settings) and selecting the desired script name from the Inbound script name field.
Once you do that, Drupal will answer calls by activating the selected script.
Note: the module voipscriptsamples.module provides a couple of sample scripts that you can use to test the system.