Calls records are being created with the wrong value in the "is human" field. It could be either true or false opposite to the expected boolean value. For example, Incidents have called users multiple times, even though they answered the phone and listened to the message Looking at Notify > Calls it looks like it is not registering that the phone has been answered.
Incorrect AnsweredBy value passed by Twilio
Log into Twilio, check the call logs and you will find the request headers passed into Servicenow. You will notice that the AnsweredBy attribute has the wrong value in Twilio for the records affected by the issue.
This value is set by the telephony provider and needs to be populated correctly for the is_human flag to work correctly in ServiceNow.
Unfortunately, there is no workaround possible from the ServiceNow side for this issue as we receive a wrong value that we cannot validate.