Ask HN: Where do virtual phone number services get the numbers from?

There are hundreds of free and paid services or just plain websites which have virtual phone numbers available for receiving SMS messages.

For example:

https://online-sms.org

https://quackr.io/temporary-numbers

(there are hundreds of those and they work, I've just tested it)

Then there are professional services such as Twilio.

Due to the abundance of those websites I assume it must be fairly straightforward to get such a virtual number yourself, but my best Google Fu didn't yield any good results.

Does anyone know how this works and would be able to shed some light on it?