Yes. You are charged for every Twilio phone number you own in your Twilio project, even when your project is suspended.
To avoid charges for your Twilio phone numbers when you are not using them, we recommend that you release these numbers. There is no way to pause the billing for a Twilio phone number.
Notice: When adding funds to reactivate a suspended Twilio project, the charges for any phone numbers you have provisioned will be listed on the month your project is reactivated. For example, if your project with 2 phone numbers was suspended with zero balance for 2 months, you would see a total of 6 phone number charges billed in the Console Usage page on the month you reactivated your account:
2 phone number charges from the 1st month of suspension
2 phone number charges from the 2nd month of suspension
2 phone number charges for the current month
For more information about Phone Number billing, see How Much Does a Phone Number Cost.