Although many Twilio customers permit their end customers to port away Twilio phone numbers from the Twilio platform, you should determine this issue with your legal counsel.  In particular, many Twilio customers permit their end customers to port away phone numbers under the following circumstances:

• Your end customer has been assigned or purchased a specific phone number, pays for the use of that phone number, and is the party that answers the phone number whenever the number is called.
• Prior to signing up for your service, your end customer used the phone number and then ported the phone number into your Twilio project.