How do I stop managing a phone number in Switchboard?

Under Settings, expand the option related to the phone number you would like to reconfigure. Click on the settings dropdown-cog (top right), and then select the Remove number from Switchboard option. This option will only be available for phone numbers currently managed in Switchboard.

At this point Switchboard will no longer handle incoming calls, messages, etc related to this number. Your phone number will still be available to you in your Twilio account and you will continue to be charged the monthly fee (by Twilio). You will need to explicitly delete the number if you no longer wish to use it.