Ask HN: Disposable Phone Number sites and legality

Recently I've seen a lot of websites that provide a disposable phone number for receiving SMS (https://freephonenum.com/) this is one I found when I googled. Most of these services are used for verification of phone numbers. I was thinking of starting a similar service but I'm not sure about the legality of such services. I know most of these website gets a lot of traffic, which tells me that there are a lot of people who want this kind of service for privacy or may be any other purpose. Different views are highly appreciated.