It turns out this is not something we can do. In order to use your Twilio number with RingCentral you would need to port it out of Twilio to RingCentral. In general, and in large part due to technical/FCC requirements, you can only use one phone number with one service provider's features.
Found this answer in Google as we currently are looking for a VOIP solution but have a few Twilio numbers. I understand it can be ported over. Does RingCentral still allow the number to send and receive SMS?