Submitting a Regulatory Bundle and getting it validated is required in some countries before you are able to use the phone number. This is only applicable in some countries, like in the UK, Australia, and Switzerland.
If you have already submitted a Regulatory Bundle and it's verified, but you can't select it when purchasing a phone number (no options), like the following, that most likely means that you need to submit an Address Bundle as well.
In the Regulatory Bundle/Address section in Settings > Phone Numbers, you'll be able to select "Address Bundle" after clicking on the +Create button.
So the Regulatory Bundle must be coupled with an Address Bundle before you can assign a phone number to it.