If you own multiple numbers, you can set up one flow that applies to every number you have; this is called a ‘Default Flow’. An SMS default flow will work for all SMS capable numbers, and a Voice default flow will work for all Voice capable numbers. If you select this option, remember that the flow will be applied to all your numbers, unless a number is already attached to another flow. 

To set up a default flow, navigate to Flow Builder and open one of your existing flows or create a new flow. Click on the gear symbol in the top-left corner. Toggle the switch to enable the default flow. Don't forget to save your changes.