There are two types of phone numbers listed within your Twilio Account, verified phone numbers and Twilio phone numbers.
A verified phone number is one that you can use as your Caller ID when making outbound calls with Twilio. This number has not been ported into Twilio and you do not pay Twilio for this phone number.
Outbound calls made from Twilio with the Caller ID set to your verified phone number will be charged to your Twilio account and not to your verified phone number.
When you receive a call on one of your verified phone numbers, the call will arrive on the device which the phone number is connected to. Twilio will not receive any incoming calls made to your verified phone number since it is not a Twilio phone number and is not configured to any Twilio application.
A verified phone number cannot be used to send Twilio SMS messages.
A Twilio phone number is one that you've either purchased through Twilio or have ported into Twilio. These phone numbers are configured to your Twilio application and, depending on their capabilities, may be used to make and receive Voice calls as well as send and receive SMS messages through Twilio. The cost of these numbers are charged to your Account Balance on a monthly basis, in addition to the traffic that is routed over these numbers.