Billing Telephone Number (BTN) is the master telephone number that is used to identify an account with a carrier.

Any number attached to the same account can be associated by using the BTN.

Depending on what type of account you have, you could have one or multiple BTNs. When porting numbers to Plivo, you want to be sure to ask your current phone provider what the BTN(s) is/are for the number(s) you wish to port.